Hey again, @Chantal787. 

I appreciate you coming back with additional information about your problem. 

To clarify, is there a payment linked to this specific invoice that you're trying to add to a project? If so, this could be the reason this is occurring in your QuickBooks Online account. 

In this article, it states that, "When you add a paid invoice to your project, the invoice disconnects from attached payment and then invoice status changes to unpaid. After you edit the invoice, you'll need to avoid accidentally charging your customer twice. "

I'll be waiting for your response. I'll be back around shortly!