Is that an original Poly 800? Wow that was my first keyboard. It was nice and portable and teamed with my Roland 505 was my first live midi bass player. I think I traded a Fender Twin for it.
So we will assume your using real midi via your interface.
Make sure you have the Tascams midi ports checked in Preferences/Midi/devices/Input.
Then you chose the Tascams midi port /OMNI as the input for the midi track.
I know this sounds dumb, but make sure you have your midi cables connected correctly the keyboard output to to the tascam input.
There also might be a setting in the Korg that activates midi output and chooses a channel.