It isn't common, but you can use the paid versions of avast for 30 days unrestricted trial, after this point you have to insert your license file. So it sounds like you haven't done this ?
When you purchased avast you should have got an email with either a link to download the license file (license.avastlic) or it would have been attached to the email. You have to save this attachment (license.avastlic) to your hard disk, somewhere you can find it again. Now locate it on your hard disk and double click it, that should automatically enter your license.