Invoicing a customer (foundation) without a vAT number

Well, for tax purposes they are a private person. There's nothing wrong with entering them that way.

For EU customers, you have to do validation of the vAT number, so you can't enter them that way, can you? That's what you mean, right?

But as a private person, it's actually fine, because that's what it boils down to for tax purposes if they don't have a vAT number.