Some API features, such as the sim transfer API feature only work with User tokens. Application token's don't have enough permissions to work.
I've put together a demo script that shows the failure.
You can see it using the workspace app token and that working fine for fetching sim data.
But then using the token for migration between workspaces is failing, because those tokens only give permissions to one workspace, not both as is needed for a migration between workspaces.
The feature requests asks that a way be provided to authenticate and get a token that has enough rights to call the sim transfer API call.