I purchased a multi-device, multi-year license for Avast Premier.
This was successfully enabled across the 3 devices for which I am licensed.
Recently, I noticed that my devices are now only running Avast Free and although the purchased license shows up in the application, when I try to select it I get ‘an unknown error occurred’.
You would need to contact the reseller, assuming this was an authorised avast reseller they should be able to resolve this.
However, if this was just someone selling licenses on-line etc. (many such sellers on ebay/amazon) this could be a hacked license which has been cancelled by avast.