Making Tax Digital for VAT was introduced in April 2019 and the functionality has come a long way since then, however, there are still sometimes issues with the steps and step 3 in particular so there are some tips on how to handle common issues;
Make sure you are using your Corporation Tax number (Company UTR) not your personal tax UTR!
Postcode entered must match the postcode HMRC holds for your business for VAT
Give it a week between your registration date and attempting to register, HMRC’s records can take a while to update!
Crunch have a help article on it too if you want to review that: [https://help.crunch.co.uk/s/article/adding-the-vat-service-to-your-business-tax-account-for-making-tax-digital-mtd-for-crunch-clients]
Hope this helps!