I ask the experts of the manager program to advise us on how not to create and save a sales invoice without selecting the customer and the items
At this stage you can not prevent creating a Sales Invoice without selecting a customer or items. However, they will got to suspense account signalling that there is a problem that must be fixed. As such regularly clear up the suspense account (is in summary page at the bottom-left).
As for Customer selection I think that eventually it should prompt to add one if not created. However for items this is not possible because you could also sell something else such as a service for whcih you only wpuld select and account and not an item. So it could never be compulsory to select an item.
You don’t need to use Item column. Just select an account in Account column instead.
However you need to use Customer field.