The asking for activation was a bug in CCleaner v6.15.10824 that was pulled from release - it has been fixed with CCleaner v6.18.10838.
That bug has been fixed and entering your key in v6.18.10838 should now work as it has always done and it won't ask again.
(I had that bug myself on 2 different machines, both are OK now).
However you now have a different issue in that CCleaner is not accepting your key and thinks that it doesn't exist.
That is a pretty specific message, not 'expired' not 'dectivated', but simply 'doesn't exist' at all.
Obviously the key does exist, support were able to quote it back to you.
Just to double check - you are copy/pasting it and not typing it in?
I do have another though that can also give a 'does not exist' error:
Make sure that you are using the same name that you registered when originally purchasing the licence.
It may be that it's the <u>name</u> you are using that 'doesn't exist' on the system because it isn't the name that you registered.</strong>
Both typos and using a name other than the registered name are common mistakes that we see.
Other than that then just why the licencing system thinks the licence doesn't exist I don't know but that would not be a file/setting on your machine, if it thinks the licence doesn't exist then it's something at the licence server end.
Only support can access that to try and see what the problem actually is.
We are aware that support are taking very long times to reply at the moment, one thing to note is to only 'reply' to the emails they have sent you.
If you send seperate, new, emails then it will slow the system down even further.
Your CCleaner should be working in Free mode until you can get a resolution.
I have flagged your problem up to the CCleaner staff.