Unauthorized changes blocked
That is your Controlled Folder Access in Windows security.
Controlled Folder Access is a heavy handed security measure that means only Windows itself can modify certain things.
It's usually off by default, so you must have turned it on.
It's always been heavy handed, it works on a 'block everything' principle.
For instance when it first came out it was on by default and it wouldn't let any 3rd party word processor save documents, only Word itself could save them.
(Took me hours to work out why I suddenly couldn't save any documents, spreadsheets, or images).
You can either turn it off again, or make CCleaner an exception.
https://www.winhelponline.com/blog/use-controlled-folder-access-windows-10-windows-defender/
You may/will find that CFA may/will also restrict other programs/apps that are not Microsofts own apps, or on their whitelist.