Getting Fn::GetAtt error in the AWS SAM template - aws-cloudformation

Question

I have declared the SNS topic and Subscription like below in my AWS Serverless Application Model template :-
MyTopic:
Type: AWS::SNS::Topic
Properties:
DisplayName: !Sub 'test-${Environment}-${AWS::AccountId}-${AWS::Region}'
Tags:
- Key: Environment
Value: !FindInMap [Environment, FullForm, !Ref Environment]
TopicName: !Sub 'test-${Environment}-${AWS::AccountId}-${AWS::Region}'
MySubscription:
Type: AWS::SNS::Subscription
Properties:
Endpoint: !Ref SubscriptionEndPoint
Protocol: !Ref SubscriptionProtocol
Region: !Ref 'AWS::Region'
TopicArn: !Ref MyTopic
And then using the SNS Topic ARN in my Lambda function's environment as following in the same template file :-
MyLambda:
Type: AWS::Serverless::Function
Properties:
Environment:
Variables:
RUNTIME_SNS_TOPIC_ARN: !GetAtt MyTopic.Arn
Outputs (in SAM template):-
MyTopic:
Description: SNS Topic for the Ingest to send notification to
Export:
Name: !Sub
- ${ExportPrefix_}:${AWS::Region}:MyTopic
- ExportPrefix_: !If
- HasExportPrefix
- !Join ['-', [!Ref ExportPrefix, !Ref Environment]]
- !Join ['-', [!Select [0, !Split ["-", !Ref "AWS::StackName"]], !Ref Environment]]
Value: !Sub "${MyTopic.Arn}:${MyTopic.Version.Version}"
MySubscription:
Description: Subscription to get messages from a topic
Export:
Name: !Sub
- ${ExportPrefix_}:${AWS::Region}:MySubscription
- ExportPrefix_: !If
- HasExportPrefix
- !Join ['-', [!Ref ExportPrefix, !Ref Environment]]
- !Join ['-', [!Select [0, !Split ["-", !Ref "AWS::StackName"]], !Ref Environment]]
Value: !Sub "${MySubscription.Arn}:${MySubscription.Version.Version}"
However, I'm getting following error :-
13:30:30 Error: Failed to create changeset for the stack: my-stack, ex: Waiter ChangeSetCreateComplete failed: Waiter encountered a terminal failure state Status: FAILED. Reason: Template error: resource MyTopic does not support attribute type Arn in Fn::GetAtt

Answer 1

An AWS::SNS:Topic returns the ARN when you use Ref
Check out the Docs on the return values.
Try with
MyLambda:
Type: AWS::Serverless::Function
Properties:
Environment:
Variables:
RUNTIME_SNS_TOPIC_ARN: !Ref MyTopic # Using !Ref

Answer 2

Recommend trying the CloudFormation Linter in VSCode to see some of these errors inline while authoring templates:
[cfn-lint] E1010: Invalid GetAtt MyTopic.Arn for resource MyLambda
[cfn-lint] E1019: Parameter MyTopic.Arn for Fn::Sub not found at Outputs/MyTopic/Value/Fn::Sub
[cfn-lint] E1019: Parameter MyTopic.Version.Version for Fn::Sub not found at Outputs/MyTopic/Value/Fn::Sub
[cfn-lint] E1019: Parameter MySubscription.Arn for Fn::Sub not found at Outputs/MySubscription/Value/Fn::Sub
[cfn-lint] E1019: Parameter MySubscription.Version.Version for Fn::Sub not found at Outputs/MySubscription/Value/Fn::Sub
It'll also point out that Value needs to be indented less in the Outputs
AWS::SNS::Topic return values
AWS::SNS::Subscription