Every so often, CC tells me that I must close my browser before it can clean, but the browser ( Avast ) is always closed down. Then, I am asked if I want to force close my browser but CC can't seem to do that ( maybe because it's already closed down! ) Help!!!
I believe that Avast Secure Browser can take some time to fully close, so if you try to clean immediately after closing it then it may still be running and still closing itself.
That could explain why you are only seeing it "<em>every so often</em>", you are being quicker to clean or the browser is being slower to close on those occasions.
Wait a bit longer and try again.
Or possibly you have the browser set to run in the background after closing it?
Look in the browser settings for "Continue running background apps when Avast Secure Browser is closed" and turn it off.
As for why CCleaner can't force it closed when asked to:
If you also have Avast AV then that has a self-defence module to stop anything from forcing Avast apps to close.
That may be what is preventing CCleaner from force closing the background tasks for the Browser.
If that is the case then you could try disabling the self-defence module from the AvastUI > Settings > Troubleshooting.
Although it seems a bit heavy handed to turn off that protection just to be able to occasionally force close the browser.