Some creations take longer to complete than others. Keep MyTunes open if the app asks you to wait and check your Library again after a few moments.
If the creation remains stuck:
Check your internet connection.
Close and reopen MyTunes.
Check whether the result appears in your Library.
Contact support with the creation type, approximate start time, and any error message shown in the app.
Do not repeatedly submit the same request while it is still processing. This can make it harder to identify the original generation.