I bought Avast 2 days ago and I believe I did what was indicated, but I got a post that it was expired. I need help, by the way, first time on forum… don’t laugh 
okay, did you do the following with your license key. Did you double-click to open it and your license will be inserted into your program automatically. Did you purchase your license at the official website, http://www.avast.com/index
Expired or Invalid ?
If expired - Check the system date, day/month/year and adjust as required.