Thank you for the quick response, @REICT!
Allow me to join in this thread and share additional insights about linking estimates to your invoices.
First, the customer you've chosen in the CUSTOMER: JOB drop-down menu and the status of your estimate if it's still open or already linked to invoices.
These are the two common reasons why you're unable to link your estimate from an invoice.
Thus, let's make sure to review the details of your estimate and the chosen customer of your estimate. This way, you'll be able to link your estimate to your customers invoice successfully.
Lastly, here's an article you can read to learn more about estimates and invoice: Create an Invoice in QuickBooks Desktop.
Feel free to leave a comment below if you have any other questions about estimates and invoice. I'll be happy to help you out.