This is an uncommon problem. If a program isn't in the list of programs under Control Panel it means that it's not installed on the computer. Try to completely shut down your PC and then start it. Check again the Programs and features section from Control Panel to see if the program is listed. If it's not, swipe from the upper right corner of the screen and in the search bar type Avast uninstall. If yuou can't find the uninstaller, try going to C:\Program Files and search for the Avast folder. That is the default location of the uninstaller. After all this, if you still can't uninsatall the program, try using CCleaner or Revo Uninstaller.
Also, you can try to download and run the Avast Uninstall utility available on the following official website.