Before Easy MTD can be used to view obligations, liabilities, payments and penalties, and submit VAT returns for a business, Easy MTD must be connected with HMRC on behalf of the business.
If you have previously connected Easy MTD with HMRC for the selected business, it is likely you are getting prompted to do so again because the connection has expired (authority is valid for 18 months then it must be renewed).
More: For further details about connecting Easy MTD with HMRC, refer to the Connecting with HMRC section of the help.