This article refers to releases and features. Depending on your workspace type, you may see "schedules" and “activities" in your workspace.
Aha! Roadmaps | Duplicate workspace and workspace line prefix error
When setting up workspaces and workspace lines, you must choose a prefix so that record IDs can be assigned. Sometimes you may realize you prefer one prefix over another and you begin to edit them.
If you get the error, "Prefix can not be one used previously" it means you've used that prefix before and you created an object that uses it as part of its ID.
You can reuse any prefix, as long as the prefix is not being used by any records (features, requirements, ideas, notes or releases). The solution is to delete the records in question and that will free up the prefix. If you have many records, then we would recommend selecting another prefix.