This error message from HMRC indicates that you have not completed the registration process for MTD for VAT, or the registration process has not yet completed.

HOW TO REGISTER

1. Register with HMRC for MTD for VAT 7 days after you have filed and paid for your last traditional VAT return

Register at least one week before your first MTD VAT Return is due if you pay your VAT by direct debit, 3 day before if not.

Register at           https://www.tax.service.gov.uk/vat-through-software/sign-up/have-software

You’ll need:         your Government Gateway user ID and password
                          the VAT number of the business that’s signing up

2. You’ll receive an email within 72 hours to confirm that you can submit returns using software that’s compatible with Making Tax Digital for VAT.

3. After you have received the confirmation email authorise the link between GoSimpleVAT and HMRC.