PC-12 Pilatus Captain reported pressurization system failure during cruise.

Narrative

During a flight from ZZZ to ZZZ1; while over ZZZ2 at FL240; an ACS (Airman Certification Standards) Low Inflow CAS message was triggered; accompanied by an audible change in the aircraft's pressurization system. Upon noticing this; I initiated the QRH procedures while the First Officer took control of the aircraft. We successfully reset the ACS.To ensure safety; we discussed our plan in the event of a reoccurrence of the CAS message. We decided if we were to get another CAS message; we would initiate an descent to 10;000 feet; regardless of whether the ACS was reset successfully; out of an abundance of caution. Approximately five minutes later; the same CAS message reappeared. We promptly instructed passengers to don their oxygen masks and proceeded to do the same. After [requesting priority handling] to the controller; we executed the descent to 10;000 feet; successfully leveling off and ensuring passenger safety. The highest cabin altitude reached during this descent was approximately 9;800 feet due to the ACS re-engaging.After addressing the passengers; it was discovered that seat 5 did not have an oxygen mask installed. The passenger was required to switch from seat 5 to seat 2 to obtain a mask during the descent.Regrettably; I made the assumption that the mask was present in the receptacle during my cabin inspection. This assumption was based on previous experiences where I have observed the red tab being unintentionally pushed into the receptacle by passengers leaning on the lids; necessitating reinstallation sometimes with the tab not being visible. Moving forward; I will ensure to visually confirm the presence of the red tab at each station to prevent this issue from occurring again.