Cross Region Copy of Aurora Snapshots
Your proposed snapshot-id is not syntactically valid.  When you tried hard-coding the value, you used a valid identifier.  

Underscores are not allowed, and the identifier can't begin with a number.

Here are the constraints on snapshot identifiers, from the API Reference:


  DBClusterSnapshotIdentifier
  
  The identifier of the DB cluster snapshot. This parameter is stored as a lowercase string.
  
  Constraints:
  
  
  Must contain from 1 to 63 letters, numbers, or hyphens.
  First character must be a letter.
  Cannot end with a hyphen or contain two consecutive hyphens.
  
  
  https://docs.aws.amazon.com/AmazonRDS/latest/APIReference/API_CreateDBClusterSnapshot.html