When I try using the licensed number as given (which is 18 characters) I get dialog back that says it should be 32 characters and therefore I am not able to install the app.
Don’t post your key here of course, but if it is a new key then it should have 20 characters including the 2-dashes.
Like this: XXXXXX-XXXXXX-XXXXXX
Are you trying to enter your key in the wrong place?
For example, are you trying to register CCleaner for Android?
I ask that because some Google keys are 32 characters long so if you try to put a 20 character key into one of those then Android will shout that it isn’t long enough.
See this guidance about registering and activating CCleaner, (including where/how to do it for CCleaner Android if you have a Pro Plus or Premium licence).
https://support.ccleaner.com/s/article/how-to-register-and-activate-ccleaner-professional?language=en_US