Unable to reference CloudFormation resource in serverless.yml. Invalid variable reference syntax for variable UserPoolId
After taking some time off the computer, I thought that the problem could be somewhere else which is what the case here.
I was using ${UserPoolId} under environment variables and realized that this is where the problem lies. I changed to the following way and the problem got solved. I was not referencing user pool ID correctly (it was referencing serverless.yml local variable named UserPoolId which didn't exist)
userPoolId: Ref: UserPoolThis was just my mistake and is now solved.
I found whilst writing my template it sometimes would do incorrect syntax highlighting and I would have to write it so that it couldn't check.
Ref: UserPool
Fn::Sub: '${UserPool}'
By swapping it with Substitute, it cannot check that the contents of the string are valid and then it will construct a string containing the Ref to the UserPool, or the user pool id