The below steps have to be done for the submit button to correctly submit the VAT with no error:
Select VAT Management.
Select MTD SUBMIT tab.
Check that the value for the VAT return is populated in the boxes at the bottom.
If these are not populated follow the below steps:
Select the VAT Detail tab and select BUILD DATA at the bottom right corner.
This will populate all the figures back in the MTD SUBMIT tab.
Once the values are populated you can then go through and do the below steps:
Select the Authorise button and fill in your Gateway details, this should populate an Auth Key there must be a key in the box to submit.
Then select Receive to pull through your obligation from HMRC, these boxes must be populated before submission too.
Check the value of the VAT boxes and then select Submit once all the above boxes are populated.