I got this message from MAXON:
In MAC Sierra or High Sierra there is a bug where the temporary serial numbers cannot be overridden with the permanent.
To get around this you must go to this file path(which is a hidden folder):
If you do not know how to unhide folders please go to the finder menu and hit GO > Go To Folder and paste that path in the folder search bar. Please make sure to replace **USERNAME** with your actual computer's username.
Before the next step, it is very important to make sure Cinema 4D is shut down.
Next, you will delete all your 20mca and 20mcn files. This is where your serial numbers are stored.
Finally, launch C4D. You will be prompted to enter your serial number again. This time use your permanent registered serial number.
We do apologize for the inconvenience. Our dev team is aware of the issue and working on a fix.
I hope this helps someone.