There are two different errors you can experience.

"Credentials" error or an "authentication" error.

Credentials Error

Make sure you're entering your correct Government Gateway User ID and Password - no typo's.

Did you check the box below the Government Gateway User ID field, before you hit the green 'Submit to HMRC' button? If not, check the box then try to submit again.

Authentication Error

If you are new to self-employment have you set up a Personal Tax Account on the .GOV website - to receive your tax return?

You must do this before you hit the "Submit to HMRC" button in the app - use the link provided below.

Registering for the Personal Tax Account only takes a couple of minutes and makes submitting your tax return possible.”

It's very straightforward.

Follow this link: https://www.gov.uk/personal-tax-account

If you have already set up a Personal Tax Account.

Customers submitting their tax return online for the first time will need to request an activation code, (which activates your account so it can receive a digital submission).

HMRC will be post the activation code to your home address, it can take up to seven days to arrive.

Once your Activation Code arrives (only valid for 10 days) log back into your Personal Tax Account add the activation code then save and log out.

Your account is now ready.

Come back to the App:-

Open the 'Tax Return' tab
Select the correct 'tax period'
Hit the 'Proceed' button
Check over your personal information then hit the green 'Submit to HMRC' button

Done that but you're still getting an error message?

Send an email to: [email protected]

Once your account is "Authenticated" and you've submitted your first return you won't have this issue submitting future returns.