The answer to 'ductless system smells bad' is that a foul odor coming from a ductless HVAC system is often caused by mold, mildew, or bacteria growth. This can happen if the system is not properly maintained and cleaned on a regular basis. Symptoms may include a musty, rotten, or unpleasant smell coming from the indoor unit.
To address this issue, you'll want to first check the air filters and clean or replace them if they are dirty. You should also clean the indoor unit's coils and condensate drain line to remove any buildup. If the smell persists after a thorough cleaning, it's best to call in a professional HVAC technician to inspect the system and determine the root cause of the odor.