Can't delete Avast!! When attempted the following note is generated. "The item “AvastCleanup” can’t be moved to the Trash because it’s open."

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Safe mode prevents any 3rd party software from activating, thus getting are out the "open" restriction.

Locate the developers uninstall instructions and follow them. You may need to do this in Safe Mode.