The error, Please supply valid RTI credentials, is because the HMRC gateway can't verify your credentials, or your PAYE reference is inactive. To check your credentials, follow the steps below.
🤓Tip: Try and log in to the HMRC gateway with your credentials, to confirm they are correct.
Click System, then click Company Details.
Click the HMRC tab, check the fields.
PAYE Reference.
Tax District Name.
Payment Reference.
Click the Year End Returns tab, check the fields.
User ID
Password
If your details are correct and the error message persists, contact HMRC to check your gateway account.