Why oh why can't I get a 26mm bar in a 26mm stem?

measure it, the stem might be 25.4

if its second hand the person before you might have overtightened it and it might be seized up, maybe put a pair of pliers in and pull the grips away in an attempt to open it up a little bit

if all else fails grab an inverse shim from your lbs, it should be pretty cheap

almost too cheap...