Adobe officially states that and later are compatible with macOS Catalina.
: Use the Freeform Gradient tool (found in the Gradient panel) to create the soft, colorful backgrounds popular in macOS-style design. adobe illustrator macos catalina
If you must use older, unsupported versions of Illustrator (like CS6) but your machine is locked onto macOS Catalina, you have limited choices: Adobe officially states that and later are compatible