If you mean they’ve sent you a code by post, that’s usually only for when you’re first setting up VAT and haven’t yet attached your VAT number to your Government Gateway account at all. If you’re already able to see your previous VAT returns under the “Making Tax Digital for VAT” section of your HMRC online account then you’re already set up on the HMRC side and you just need to do the connection and configuration on the QuickFile side.
Under Reports -> HMRC you may already have your tax account connected, if not then use the “connect” button, select the “organisation” option (not “individual”), and it should redirect you to the Government Gateway to log in, then back to QuickFile to say the account is connected.
Then under Reports -> VAT Returns, press the “settings” button, then make sure that the two checkboxes for “enable online filing” and “use MTD features” are both ticked (the “use MTD” box only appears once the “enable online” box has been ticked"), and in the drop-down box for HMRC account select the account you just connected. Once you save these settings you should be able to go back to Reports -> VAT Returns -> Create and if it shows you a list of periods then you’re good to go.