Greetings, @Chantal787.
It's great to see a new face on this thread. Let's work together to help you add an existing invoice to a project within your QuickBooks Online account.
If the invoice is unpaid, review the steps below to add the invoice to the project:
For further details, check out this guide: Add existing invoices to QuickBooks Online Projects.
I hope this helps and does the trick for your business. I'm only a reply away if you need me. Take care!