Maxon occasionally removes old installers. Contact Maxon support directly. They provide legacy download links via customer support tickets.
Log in to see if R18 is listed under your historical purchases. Download Cinema 4d R18
If you do not have a license for R18, consider these legal free options: Maxon occasionally removes old installers
installer into the terminal and press Enter to bypass security restrictions. R18 Key Features MoGraph Updates: Download Cinema 4d R18