Good afternoon, @NVO415. 

Thanks for coming to the Community with your question about the Pay Invoice button. 

To clarify, is this happening to all invoices or only certain ones with a particular customer on them etc.? 

In the meantime, check out this guide: Record invoice payments. 

This will help us determine the next steps you need to take to get this button back as soon as possible. I'll be waiting for your response!