Open choices in the program

I want to open choice that let me able to use any account in the sale and buy invoice .
why should I put specific name for supplier in buy invoice ? and specific name for customer in sale invoice?

your question is not clear.
if you do not have any supplier or customer then you should be doing a cash based business where invoices are not involved. in this case use the Payments & Receipts tab for both purchases and sales. read below guides.

Because invoices are posted to subsidiary ledgers under Accounts receivable (for sales invoices) or Accounts payable (for purchase invoices). Without the customer’s or supplier’s name, there is nowhere to post them.