If you try to install the latest Arnold 4.7.x on Cinema 4D R21, you will face a hard stop. Autodesk/Solid Angle dropped support for R21 and R22 after C4DtoA 4.4.0. R23 saw limited support in 4.4.1, but 4.4.0 remains the most tested.
: This specific update maintains full compatibility with Cinema 4D R21, R22, and R23 , which is essential for artists who have not yet migrated to the Subscription-only releases. arnold render 440 for cinema 4d r21 r22 r23 r updated
You can download the specific legacy installers for these versions directly from the Autodesk Arnold Downloads page. 4.7.7 - Arnold for Cinema 4D If you try to install the latest Arnold 4