I have been trying to submit online vat, I have used my Gateway credentials, but still receive this message
“Vat return submission to HMRC failed.
By Gateway : Authentication Failure. The supplied user credentials failed validation for the requested service.”
Are you copying the credentials across from somewhere? Most common issue is copying some errant HTML or hidden characters as well. Try pasting into a text editor (Notepad or similar) first to clear any formatting or type them in rather than copying and pasting.
The other thing that will prevent you submitting VAT returns is if you’re not registered with the government gateway to use that service.
If you log into your Government Gateway account you should see an option to submit a VAT return on your main control panel (see below). If that option isn’t there you will need to add this as a service in your Gov Gateway account.