User wants to change the project for an expense entry after making a payment for it in the Bill Payments screen.
If you have created a bill payment for an expense entry as a part of a vendor bill or as a reimbursable expense, you cannot change its project in the Expense Entries screen, nor can you change it in the associated vendor bill. Core allows very limited editing for them in order to keep the accounting for the bill payment consistent. So you first need to delete the bill payment, then delete the related invoice (if any) and then change the project for the expense entry. You can then re-create the whole thing with the correct project. Check out this article for details on changing projects on paid vendor bills.