Studio Operations prevents you from deleting a person when that person is connected to a sale.
A buyer on sale paperwork is part of the transaction record. Deleting that person would leave the sale and its documents without the buyer information they depend on.
What to do instead
If the person is attached to a sale, edit their contact details instead of deleting the record.
If the wrong person was attached to the sale, or you believe the transaction record itself is incorrect, do not delete surrounding records to make the screen look right. Start with the sale record and use Guided Help before changing related records.
This protects the sale history, the artwork’s selling state, and the paperwork from accidental damage.