Publish AWS Serverless Application - Failed to create CloudFormation change set: Template Error

[ibbyu]

Describe the bug

... Waiting for change set to be reviewed
Error getting status of change set: Failed to create CloudFormation change set: Template format error: The Value field of every Outputs member must evaluate to a String and not a List

To Reproduce
Steps to reproduce the behaviour
example:

1. Create new Serverless Application (Web Api)
2. Add some parameters & mappings to serverless.template. Example:

Parameters:
  Environment:
    Type: String
    AllowedValues:
    - dev
    - prod
    Description: The environment. Default is dev.
    Default: dev
  SubDomainName:
    Type: String
    Description: The subdomain for Api. Default is api.
    Default: api
Mappings:
  DomainMap:
    dev:
      Name: dev.example.com
    prod:
      Name: example.com

3. Update Output value to sub values found in DomainMap using FindInMap.
   (This gets highlighted as an error 0: Unresolved tag: !FindInMap). Example:

Outputs:
  ApiURL:
    Description: Api URL
    Value:
      Fn::Sub:
          - "https://${SubDomainName}.${DomainName}"
          - DomainName: !FindInMap [DomainMap, !Ref Environment, Name]

4. Right click project -> Publish to AWS Lambda -> Go through options & Publish
5. Error:

Error getting status of change set: Failed to create CloudFormation change set: Template format error: The Value field of every Outputs member must evaluate to a String and not a List

Expected behavior
Should deploy successfully. This exact same template works when using sam build & sam deploy

Screenshots
Error when using VS GUI:

Successful deployment after using sam build & sam deploy

Toolkit Logs

Development System (please complete the following information):

• Windows Version:
• Visual Studio Version: 17.9.6
• AWS Toolkit for Visual Studio Version: 1.52.0.0

Additional context
This exact same template works when using sam build & sam deploy
(Update the Handler to match your project)

Description: An AWS Serverless Application that uses the ASP.NET Core framework running
  in Amazon Lambda.
Parameters:
  Environment:
    Type: String
    AllowedValues:
    - dev
    - prod
    Description: The environment. Default is dev.
    Default: dev
  SubDomainName:
    Type: String
    Description: The subdomain for Api. Default is api.
    Default: api
Mappings:
  DomainMap:
    dev:
      Name: dev.example.com
    prod:
      Name: example.com
Conditions: {}
Resources:
  AspNetCoreFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: test::test.LambdaEntryPoint::FunctionHandlerAsync
      Runtime: dotnet8
      CodeUri: ''
      MemorySize: 512
      Timeout: 30
      Role: null
      Policies:
      - AWSLambda_FullAccess
      Events:
        ProxyResource:
          Type: Api
          Properties:
            Path: "/{proxy+}"
            Method: ANY
        RootResource:
          Type: Api
          Properties:
            Path: "/"
            Method: ANY
Outputs:
  ApiURL:
    Description: Api URL
    Value:
      Fn::Sub:
        - "https://${SubDomainName}.${DomainName}"
        - DomainName: !FindInMap [DomainMap, !Ref Environment, Name]

[rbbarad]

@ibbyu Thanks for bringing this issue to our attention. I've reproduced this on my end - there seems to be an issue with the way we are parsing the yaml formatted template file. We are currently investigating a fix for this.

In the meantime as a workaround, try converting the serverless.template file from yaml to json. Using the example you provided, I've confirmed that a json serverless.template file deploys successfully.