Pedal screw comes loose after about a mile now matter how much I tighten it

Sometimes by riding with a loose square-taper crank arm the hole where the bottom bracket spindle fits in (where the bolt attaches) gets rounded out and won’t press-fit back on right. You likely need a new left crank arm. Should not cost a lot to replace- there should be a length stamped on it some where – like “170mm”.