AWS CloudFoundation Template error: Unresolved resource [DBInstance] YAML format

Question

Problem

Using AWS CloudFormation YAML to create resource stack with EC2, etc and RDS database, etc.

Template format error

Format resource DBInstance is location of problem in YAML script

Template format error: Unresolved resource dependencies [DBInstance] in the Resources block of the template

Scope of where error resides

  DBInstanceID:
    Default: DBInstance
...
...
...
  MyRDS:
    Type: 'AWS::RDS::DBInstance'
    Properties:
      DBInstanceIdentifier: !Ref DBInstance
      DBName: !Ref DBName
      DBInstanceClass: !Ref DBInstanceClass
      AllocatedStorage: !Ref DBAllocatedStorage
      Engine: MySQL
      MasterUsername: !Ref DBUsername
      MasterUserPassword: !Ref DBPassword
      SecurityGroupIds:
        - !Ref EC2SecurityGroup
      KeyName: !Ref KeyName

CloudFormation Template with Error

AWSTemplateFormatVersion: 2010-09-09

Parameters:
  KeyName:
    Default: TestKeyPair
    Description: Name of an existing EC2 KeyPair to enable SSH access to the instance
    Type: AWS::EC2::KeyPair::KeyName

  SSHSecurityGroup:
    Description: SecurityGroup that allows access to the instance via SSH
    Type: AWS::EC2::SecurityGroup::Id

  DBInstanceID:
    Default: DBInstance
    Description: My database instance
    Type: String
    MinLength: '1'
    MaxLength: '63'
    AllowedPattern: '[a-zA-Z][a-zA-Z0-9]*'

  DBName:
    Default: mydb
    Description: My database
    Type: String
    MinLength: '1'
    MaxLength: '64'
    AllowedPattern: '[a-zA-Z][a-zA-Z0-9]*'

  DBInstanceClass:
    Default: db.m5.large
    Description: DB instance class
    Type: String

  DBAllocatedStorage:
    Default: '50'
    Description: The size of the database (GiB)
    Type: Number
    MinValue: '20'
    MaxValue: '65536'

  DBUsername:
    Type: String
    Description: Master username for the RDS instance
    Default: admin

  DBPassword:
    Type: String
    NoEcho: true
    Description: Master password for the RDS instance
    Default: password

Resources:
  VPC:
    Type: AWS::EC2::VPC
    Properties:
      CidrBlock: 10.0.0.0/16
      EnableDnsSupport: true
      EnableDnsHostnames: true
      Tags:
      - Key: Name
        Value: VPC1

  InternetGateway:
    Type: AWS::EC2::InternetGateway
    Properties:
      Tags:
      - Key: Name
        Value: VPC Internet Gateway

  AttachGateway:
    Type: AWS::EC2::VPCGatewayAttachment
    Properties:
      VpcId: !Ref VPC
      InternetGatewayId: !Ref InternetGateway

  PublicSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.1.0/24
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: Public Subnet 1

  PrivateSubnet1:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.2.0/24
      MapPublicIpOnLaunch: false
      Tags:
        - Key: Name
          Value: Private Subnet 1

  PublicSubnet2:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.3.0/24
      MapPublicIpOnLaunch: true
      Tags:
        - Key: Name
          Value: Public Subnet 2

  PrivateSubnet2:
    Type: AWS::EC2::Subnet
    Properties:
      VpcId: !Ref VPC
      CidrBlock: 10.0.4.0/24
      MapPublicIpOnLaunch: false
      Tags:
        - Key: Name
          Value: Private Subnet 2

  PublicRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
        - Key: Name
          Value: Public Route Table

  PublicRoute:
    Type: AWS::EC2::Route
    Properties:
      RouteTableId: !Ref PublicRouteTable
      DestinationCidrBlock: 0.0.0.0/0
      GatewayId: !Ref InternetGateway

  PublicSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet1
      RouteTableId: !Ref PublicRouteTable

  PublicSubnetRouteTableAssociation2:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PublicSubnet2
      RouteTableId: !Ref PublicRouteTable

  PrivateRouteTable:
    Type: AWS::EC2::RouteTable
    Properties:
      VpcId: !Ref VPC
      Tags:
      - Key: Name
        Value: Private Route Table

  PrivateSubnetRouteTableAssociation1:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PrivateSubnet1
      RouteTableId: !Ref PrivateRouteTable

  PrivateSubnetRouteTableAssociation2:
    Type: AWS::EC2::SubnetRouteTableAssociation
    Properties:
      SubnetId: !Ref PrivateSubnet2
      RouteTableId: !Ref PrivateRouteTable

  #EC2 Instances
  EC2Instance1:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t2.micro
      SecurityGroupIds:
        - !Ref EC2SecurityGroup
      SubnetId: !Ref PublicSubnet1
      KeyName: !Ref KeyName
      UserData: 
        Fn::Base64: !Sub |
          #!/bin/bash
          yum update -y
          yum install -y httpd
          systemctl start httpd
          systemctl enable httpd
          #echo "<h1>Hello from Region us-east-2b</h1>" > /var/www/html/index.html

  S3Bucket:
    Type: 'AWS::S3::Bucket'

  # EC2 and ALB Security Groups
  ELBSecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: ELB Security Group
      VpcId: !Ref VPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: 80
        ToPort: 80
        CidrIp: 0.0.0.0/0

  EC2SecurityGroup:
    Type: AWS::EC2::SecurityGroup
    Properties:
      GroupDescription: !Ref SecurityGroupDescription
      VpcId: !Ref VPC
      SecurityGroupIngress:
      - IpProtocol: tcp
        FromPort: 80
        ToPort: 80
        SourceSecurityGroupId:
          Fn::GetAtt:
          - ELBSecurityGroup
          - GroupId
      - IpProtocol: tcp
        FromPort: 22
        ToPort: 22
        CidrIp: xxx.xxx.xxx.xxx

  S3Bucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      BucketName: 'myBucketV4'

  S3BucketPolicy:
    Type: 'AWS::S3::BucketPolicy'
    Properties:
      Bucket: !Ref S3Bucket
      SecurityGroupIds:
        - !Ref EC2SecurityGroup
      KeyName: !Ref KeyName
      PolicyDocument:
        Statement:
          - Effect: Allow
            Action: 's3:*'
            Resource: !Sub 'arn:aws:s3:::${S3Bucket}/*'
            Principal:
              AWS: !Sub 'arn:aws:iam::${AWS::AccountId}:user/@matthew'

  EC2TargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      HealthCheckIntervalSeconds: 30
      HealthCheckProtocol: HTTP
      HealthCheckTimeoutSeconds: 15
      HealthyThresholdCount: 5
      Matcher:
        HttpCode: '200'
      Name: EC2TargetGroup
      Port: 80
      Protocol: HTTP
      TargetGroupAttributes:
      - Key: deregistration_delay.timeout_seconds
        Value: '20'
      Targets:
      - Id: !Ref EC2Instance1
        Port: 80
      UnhealthyThresholdCount: 3
      VpcId: !Ref VPC

  ALBListener:
    Type: AWS::ElasticLoadBalancingV2::Listener
    Properties:
      DefaultActions:
        - Type: forward
          TargetGroupArn: !Ref EC2TargetGroup
      LoadBalancerArn: !Ref ApplicationLoadBalancer
      Port: 80
      Protocol: HTTP

  ApplicationLoadBalancer:
    Type: AWS::ElasticLoadBalancingV2::LoadBalancer
    Properties:
      Scheme: internet-facing
      Subnets:
      - !Ref PublicSubnet1
      - !Ref PublicSubnet2
      SecurityGroups:
        - !GetAtt ELBSecurityGroup.GroupId

  MyRDS:
    Type: 'AWS::RDS::DBInstance'
    Properties:
      DBInstanceIdentifier: !Ref DBInstance
      DBName: !Ref DBName
      DBInstanceClass: !Ref DBInstanceClass
      AllocatedStorage: !Ref DBAllocatedStorage
      Engine: MySQL
      MasterUsername: !Ref DBUsername
      MasterUserPassword: !Ref DBPassword
      SecurityGroupIds:
        - !Ref EC2SecurityGroup
      KeyName: !Ref KeyName

Outputs:
  EC2InstanceId:
    Description: InstanceId of EC2 instance
    Value: !Ref EC2Instance1

  S3BucketName:
    Description: S3 bucket
    Value: !Ref S3Bucket

  DBInstanceId:
    Description: DBInstanceIdentifier of RDS instance
    Value: !Ref DBInstance

Answer 1

Instead of in Parameters

  DBInstanceID:

it should be

DBInstance: