Tires don't hold air, tubes hold air. Tires hold tubes. If it's got a tube and you can't get it to hold air, take the tube out and inspect it. Just FYI, if it's a tubeless setup, the valve stem bolts to the rim and you must have contact all the way around both beads before air will stay in. I ran tubeless for a season with no sealant, so sealant may be recommended but is not required.