https://help.gosimpletax.com/hc/en-gb/articles/115000024370-Error-message-1046-Authentication-Failure-The-supplied-user-credentials-failed-validation-for-the-requested-service

https://help.gosimpletax.com/hc/en-gb/articles/115000024370-Error-message-1046-Authentication-Failure-The-supplied-user-credentials-failed-validation-for-the-requested-service

The error message you have received is sent back from HMRC when the HMRC ID and password provided do not match their records. In order that you can submit your tax return when you are presented with question asking if you have a HMRC ID and password please choose the NO option. This will bypass the need to enter in your credentials and file using the SimpleTax file only agent credentials. We would also recommend that you check you HMRC credentials and if necessary reset your password.