HMRC guidance for Making Tax Digital (MTD) for VAT, advises that before you submit your first MTD VAT Return you must authorise your software. 

In Sage 50 Accounts, you do this as part of your VAT Return submission. As you follow through the steps to submit your MTD VAT Return, a web page appears asking you for Authority to interact with HMRC on your behalf.

To grant authority, click Continue.