I just rejoined and bought ccleaner pro copied activation key cut and pasted, tried 4 times. I definitely got the key right. The key does not work.
For security reasons we can't deal with licence key issues on the open forum - you need to contact support.
Email them at support@ccleaner.com and explain your problem, they are pretty quick to sort things out.
It's best if you use the same email address that you used when purchasing, so that they can quickly identify your order.
Also https://support.piriform.com/hc/en-us/articles/204043844-How-to-register-and-activate-CCleaner-Professional for licence registration tips.
Note that as well as the licence key, entering the name exactly matching the one in your registration information is also important. For example, if you gave your name as "John Ronald Reuel Tolkien", then "John Tolkien" won't work.