Sometimes, when you try to create a new release, a error message says that "You need to choose a Version Number that's not already been used." But you cannot find a release with that version number.
That means that the release with that version number has been deleted.
You need to go into Administration > Project History Changes, and either:
1) Revert the deletion and thereby have the version number available,
2) Purge the delete so that it is permanent.
Now you can reuse the version number.