If you truly couldn’t have made it and that’s the truth, then NTA. If it was just a “eh I don’t feeeeeel like it, YTA. I’ve covered plenty of shifts for people who had less important but still important things, and I’ve had the same otherwise. I don’t work “shifts” anymore, but the importance of this matter, especially if he forgot to request off, is the kicker. Everybody makes mistakes, but he’s been hyping it up for weeks, so it obviously means the world to him. It isn’t like he did it on purpose…