Im trying to make some invoices 'Mark as Paid' but it wont let me giving me a popup to say it's an error and a giving different invoice number all I can press is dismiss.

Thanks for following up with that screenshot, basement3productions.

I want to make sure you're able to mark invoices as paid in your QuickBooks Self-Employed account.

Pop-up errors in QuickBooks Self-Employed when marking invoices as paid are often caused by browser cache issues, using outdated browser versions, or pending app updates. Were you able to try the steps to view QuickBooks in a private/incognito window mentioned by my colleague above? If so and you're still running into the error, here are a few other troubleshooting steps you can try:

• Re-record Payment: If the invoice isn't updating to "paid," try navigating to the All Sales tab, finding the payment, and ensuring it is correctly applied to the invoice.
• Check for Software Updates: If using the mobile app, ensure it is updated to the latest version to prevent bugs.
• Check for Inactive Payment Methods: If the error is related to payment types, ensure the method used is not set to inactive in settings. 

Please give this a go and let me know how things look afterward. I'll be here to help if you have any other questions.