Then your screen with avast 7 should look like this one (attached image), when you click the Insert license key.
If this isn’t the same or similar screen I believe you have a bigger problem, if it doesn’t say Registration, it could be a corrupt update and you now appear to have an avast Pro trial version.
Is there a tab for Subscription ?
If so that confirms my suspicions above. You will probably have to do a clean reinstall of avast free, more on how to do that if it is required.