Deploy with Release_command not working

Hey, We have an app running fine, we decide to add a release_command for migration.
Then started getting deployment error

Error release command failed, deployment aborted

I have even tested echo in the release but still nothing

[deploy]
  release_command = "echo  release"

I have also added a simple script in /.fly.scripts/migrate.sh

Is there any special thing needed to be done to use the release command ?
@kurt