Unable to register in version 5.72.7994

I go to option and about then click on register but nothing happens. the button changes when I hover over it, when I click the button changes color, but no register windows opens.

Please help

Is this before or after you have entered your licence key and name?

You need to enter you key/name before clicking the 'Register' button.

I don't know why the button would not function but the first suspicion would be that the install may not have been complete.

Try uninstalling your CCleaner then downloading the 'Slim' installer from here, install it and then retry registering by clicking Options>About>Upgrade to Pro.

https://www.piriform.com/ccleaner/builds

See this for information about problems when registering CCleaner:

https://support.piriform.com/hc/en-us/articles/204043844–Problems-activating-or-registering-CCleaner-Professional