Avast tells me I have several add ons I should get rid of. I go through the procedure and after about 10 minutes of what appears it searching and removing the add-ons it tells “Failed to perform changes An unknown error occurred.” What do I have to do to remove the unwanted add-ons. Can anyone tell me why the procedure I’ve used does not work.
Welcome to the forum. 
You’ll probably get better help by posting your question in the proper forum:
https://forum.avast.com/index.php?board=54.0
And provide details.