The error, please supply valid RTI credentials, is mostly commonly caused by a brief disconnection of your internet, this is more common if you company uses VPN's. Attempt the submission again, if the error still persist, try the steps below.
Click System, and then click Company Details.
Click the Year End Return tab.
🤓Tip: To confirm your credentials are correct, use the user id and password to login to the HMRC gateway.
Ensure your credentials are entered correctly, click Save and Close