i have the free for 2 yrs. got tired of the pop-ups, upgraded now i can install.
general search proves this is a known issue. >:(
If you add your license, it should automatically start the update process.
Or, you can do a Clean Install of Avast:
https://goo.gl/4Ptzkf
If you need additional help with the Clean Install, watch this:
https://youtu.be/p-h3myRD51A