Serverless: Your serverless.yml has an invalid value with key: “Ref”

Serverless Framework
1

While installing serverless with following command sls plugin install -n serverless-alexa-skills --stage dev

I am getting an error like Your serverless.yml has an invalid value with key: "Ref"

The following is my sample serverless.yml file

plugins:
- serverless-webpack
- serverless-s3-sync
- serverless-plugin-git-variables
- serverless-alexa-skills

functions: ${file(./deploy/${opt:stage}.yml):functions}
resources: ${file(./deploy/${opt:stage}.yml):resources}
custom: ${file(./deploy/${opt:stage}.yml):custom}

outputs:
DialogflowFunctionArn:
Value:
  Ref:

Need some help here.got blocked here.