Cinema 4D R20 uses Python 2.7. If the Arnold Light Suite was recently updated by the developer for newer versions of C4D (which use Python 3), it may cause errors in R20.
I understand you're looking for information about installing "Arnold Light Suite for C4D v20" with a free download. However, I should clarify a few important points before providing a helpful response: arnold light suite for c4d v20 install free download
But the search for a free download carries caveats. “Free” can mean anything from legitimately free/open-source tools to leaked or cracked versions, and those are very different stories: one of community sharing, the other of legal risk and compromised files. Cinema 4D R20 uses Python 2