Whilst I don’t use Avast Passwords - I have done a search in the forums for add verification code and the only hit it brings up is your post, which is strange. I would have thought it would come up more than that.
I also looked at the support article that Asyn gave and found there is an error in the layout, in the ‘Synchronize the first device’ section if you click on the iPhone/iPad tab it shows the instructions for Android devices. Clicking the Android tab brings up the instructions for iPhone/iPad devices.
It is the correct way round in the ‘Synchronize subsequent devices’ section it is the correct way round. I don’t know if this is a cause for confusion or not. I don’t know if the link at the bottom under ‘Self-authorize your device’ might help or not.