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This problem is actually more broad in that you cannot concatenate a string with an output at all.
The following will fail as well.
```logGroupName = '/aws/vpc/' + vpc.id + '/flows/reject'```

```logGroupName = '/aws/vpc/' + vpc.id + '/flows/reject'
    TypeError: must be str, not Output
    error: an unhandled error occurred: Program exited with non-zero exit code: 1```

did you try
```Output.concat('/aws/vpc/', vpc.id, '/flows/rehect')```