You can't delete a base copy. You need to understand how it works before doing dangerous operations.
A base copy exists if there's a snapshot. If you have 0 snap and still base copy, it could be either:
because the coalesce isn't finished
because the coalesce doesn't work
In both case, you have to take a look at your SMlog. If you have no idea about it, you should either get pro support or start to read the doc in more details 🙂
You can start here: https://xcp-ng.org/docs/storage.html#coalesce