If you are not set up properly for MTD and you try to access HMRC's list of your VAT returns (or submit a VAT return), HMRC will return an error code to your VT application.
Unfortunately, HMRC return the same error code in virtually all circumstances. This can make the error hard to track down, especially if you are acting as an agent.
When the error is received by VT, VT displays a message that directs you to one of the following pages on its web site, depending on whether the agent box is ticked in the Grant Authority dialog in VT:
These pages are effectively a list of the things you need to check.