Are you sure this function should work? Can you assign a chord from MIDI Part? I’m not sure about it. You can do so from MIDI Input for sure. But from a MIDI Part…?
Agree with AP1. You don’t bring a MIDI pattern (.mid) file over into Chord Pad. You make a midi segment as a MIDI Loop. Its a different file type. You can export a midi segment as loop under File–>Export → MIDI loop.