It may just have been a temporary server error. so try again.
If you are still having problems then as the message says the Support team will be able to help.
Send them an email at support@ccleaner.com explaining the problem.
(Or there is also a 'Contact Us' button at the bottom of the article I linked above).
It's best if you use the same email that you used when purchasing.
You'll get an automated reply and then someone should get back to you.
Normally they are quick and get to you within the hour, it may be a bit slower over the holidays.
(EDIT - Last we heard there had been a rush of support requests yesterday and it may take a day to get a reply if they are still busy).
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It may just have been a temporary server error. so try again.
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There have been periodic bursts where we have had a higher than usual rate of activation issues connecting to the licensing server starting around 03:30 am GMT on December 29th. As of 15:30pm this afternoon though (about 3 hours ago as of the time of writing) we seem to be back to normal again after our engineers gave the infrastructure some strategically placed kicks.