The HMRC credential validation is indeed one of the most complex and time consuming aspects of filing a tax return. So please bare with me testing the 4 possible reasons for HMRC's declining your submission.
you accidentally copied the credentials with an error more the 3 times and HMRC locked your submission for 2 hours (you can wait 2h and then try again) ** If your account's password has more than 12 characters, you should only use the first 12
access https://online.hmrc.gov.uk/home/services - “Services you can add” in the menu on the left, then enrol for Self-Assessment (this will check if you have the Self-Assessment active within HMRC services)
test your UTR here (know that SimpleTax does not support Partnership UTRs): http://www.hmrc.gov.uk/tools/payinghmrc/sa.htm
HMRC expects a different tax return from you (Partnership, Trusts...or even different dates)
You can contact HMRC services at 0300 200 3310 or https://online.hmrc.gov.uk/information/helpdesk#onlineServices
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