If you've assigned your keyboard in MainStage in Layout mode and you have keyboard keys without sound, you'll want to double check if you perhaps assigned some drum pads, buttons, knobs, or faders from the mpk261 to something onscreen in Layout mode? Oftentimes controllers will send midi note data for some/all controls especially drum pads, so if you have assigned a pad or other control that is sending identical midi data as your keyboard then MainStage will override the note input because it interprets that note as assigned to the onscreen button, fader, etc..
If that’s the case you’ll need to remove the Assignment in Layout mode and then check the keyboard’s user manual for tips on making those particular controls send midi cc data instead of note data.