Can't Add Permission for GSI

I’m trying to add a permission for a Global Secondary Index on one of my tables. I’m adding the following JSON to my cloudformation resource file:

              "Fn::Join": [
                "", [
                  "arn:aws:dynamodb:", {
                    "Ref": "AWS::Region"
                  ":${self:custom.accountNo}:table/", {
                    "Ref": "user/index/*"

Whenever I do this serverless spits back an error stating:

 Template format error: Unresolved resource dependencies
 [user/index/*] in the Resources block of the template

I’m not sure why this is happening. Any help is appreciated.

To answer my own question I had to create a resource on it’s own outside of the other resource and enter a more static arn.

        "Effect": "Allow",
        "Action": [