For some reason I'm having issues with side chaining. For example, I insert a compressor on my snare bus, enable external side chain, try to route my kick bus via send, but I'm not seeing it as a send option. Am I missing something?
Did you enable Sidechain at the top of the plugin GUI also?
@ michaelgalvan answer: That's awkward. Maybe share the name of the 3rd party VST so people owning it can check for similar behavior.
Another thing to try is start a new song with 2 tracks and test if it works there.