You must have at least one, or you are not in business. Remember, cash accounts include bank accounts. If your software still separates them, you are far out of date. Update.
This is your problem. You can’t create a sales invoice without first defining the customer and choosing a customer from a list. Maybe you updated from a really old version that would let you do that and now those transactions are blank. Try drilling down on the balance of a cash or bank account and see if you can recover things that way.
If you are not selling on credit to defined customers, see this Guide: