You can use older versions of AE if you would like. You're Creative Cloud subscription includes CS6, CC, CC 2014, CC 2015, CC 2015.3, and CC 2017 - all of which can be installed alongside each other without issue. This page explains how to install older versions.
Alternatively, you could transcode the footage you're trying to key into a production codec. Use AME to turn your shots into Cineform, DNxHD, or something similar, and then try to use Keylight on that.
Hi dsa sdsa,
Is the original footage from a Canon 5D MKIII or similar Canon DSLR?