Good afternoon, @Nacho978. 

I appreciate you coming back and trying the steps my colleague provided. Since the instructions didn't work to resolve the problem, let me give you more insight on capturing signatures in QuickBooks Online.

To clarify, the Get signature option will only appear after enter all required details and then saving the sales form. Here's a guide that instructs you on how to capture signatures on an invoice or estimate. 

If the issue persists, I suggest contacting our Customer Support Team for further assistance.

Please, keep me updated on how the call goes or if you're able to capture signatures in your QuickBooks Online in the end. We're always here to have you back. Take care!