You’ll need to discuss this with whomever is holding the supervisory lock, or with Apple.
Seems you might have acquired a lost or stolen Mac, or one that was not unenrolled when transferred to you.
It’s not going to be particularly useful to a buyer with that lock set, and it’ll keep showing enrollment status.
There’s nothing we can do to help you remove that. Again, check with whoever is holding the enrollment.
If you can prove your original purchase sufficient to convince to Apple of your ownership (original purchase receipts, etc), then they will consider removing the supervisory lock.