When you first file your VAT return for either yourself or on behalf of a client you have to grant authority to HMRC, this authority only lasts for an 18 month period so if when you are trying to file a VAT return you again get the message it simply means that the 18 month period has lapsed and you will need to re grant permission.

When the HMRC page comes up on screen simply scroll to the bottom of the page and click the green continue button and follow the on screen prompts.

If you filing as an agent you need to use your Agent Services Account (ASA) details to grant the authority, if filing as an individual business it will be your HMRC log in details linked to your Making Tax Digital for VAT Account.