I need to grant access to “Terms” to a custom role. I already added Accounting Lists permission with the Edit level under Setup. However, the user is still unable to edit the terms on either the customer record or the sales order record. Please let me know what I am missing.

they must have at least edit permission for SO and customer. They dont need permission for accounting list in order to update SO or Customer record unless they want to create new terms.

Yes, the role has edit permission for customers, and SO records.