It is AWS tutorial: https://aws.amazon.com/ru/serverless/use-sqs-as-an-event-source-for-lambda-tutorial/ The implementation scenario is build on a principle that all data are mandatory encrypted based on the sensitive nature of the application.
My application architecture is: Frontend(443)->AWS Lambda-> SNS -> SQS -> AWS Lambda.
Incoming data from outside into AWS Lambda is encrypted by https protocol. Do I need to use encryption in Lambda->SNS->SQS->Lambda processing?
Is internal network of AWS unsafety?
You don't need to but it just enhances the security that your application is already using.
However, regarding AWS services network transit unless you're using VPC endpoints communication from your application to that service will traverse the internet.
AWS actively encourages you to encrypt communication where possible, although it is considered fine between the ELB and the target assuming you have locked down network access.
The internal network of AWS is built for security, but this does not prevent someone gaining access to the network you've setup in AWS if there are any backdoors. By having HTTPS at least you can ensure that traffic between is encrypted in transit.