Please don’t hesitate to use search first. There are already many topics with the same questions.
https://discuss.frappe.io/t/solved-expense-or-difference-account-is-mandatory/1048
https://discuss.frappe.io/t/solved-unable-to-create-pos-purchase-expense-or-difference-account-is-mandatory/11372
https://discuss.frappe.io/t/solved-expense-or-difference-account-is-mandatory-for-item/7703
https://discuss.frappe.io/t/solved-problem-in-stock-delivery-note-when-pressing-submit-botton/6395