Have a look here, may help you, unless you provide a string to arn, the resource build in serverless assumes you’re trying to reference a lambda function to make the authorization, and can only build that kind of authorizer for you: https://github.com/serverless/serverless/blob/master/lib/plugins/aws/package/compile/events/apiGateway/lib/authorizers.js#L27
I managed to dynamically build an Authorizer reference like this: