SwaggerUI/YAML - should NOT have additional properties additionalProperty: requestBody

Designing an API using editor.swagger.io I find myself unable to add a requestBody attribute, getting an error I cannot address:

Schema error at paths['/projects/{projectId}/user'].post
should NOT have additional properties
additionalProperty: requestBody
Jump to line 91

I don't understand what I'm doing wrong, especially after looking at the requestBody documentation. Research has brought me nothing other than the tendency for errors to be misleading.

EDIT: From what the answers here have shown, it looks like the editor is supposed to use OpenAPI 2.0, but actually expects 3.0 while returning errors for both. I'd use some help on what to use, given that I've included a

swagger: "2.0"

line at the beginning of the document. While testing with openapi: 3.0.0 as shown by @Mike in his answer, I just get more errors about allowed additional properties.

Here's what's generating the error, line 91 being post: .

/projects/{projectId}/user:
      post:
        tags:
        - projects
        summary: Modify project user.
        operationId: modifyProjectUser
        parameters:
        - name: projectId
          in: path
          description: ID of the project
          required: true
          type: integer
          format: int32
        requestBody:
          content: 
            application/json:
              schema:
              $ref: '#/definitions/User'
        responses:
          "200":
            description: Successful operation
            schema:
              type: array
              items:
                $ref: "#/definitions/User"
        security:
        - api_key: []

Solution

I got clarifications from an external source, so here's what I've learned:

Specifying swagger: 2.0 also means that the OpenAPI Specification 2.0.0 is expected by the editor, whereas I thought it used OAS 3. I'm still unsure about why in: body did not work in the first place but I've added quotes around "body", which made the error disappear. Then I tried removing the quotes and it worked fine.

The editor doesn't seem very reliable when it comes to error reporting.