A “moral impossibility” is something that is not physically a totally impossible thing to do, but which is so difficult that it is practically impossible (or it’s a really bad idea).
For example, let’s say that an evil government said that they weren’t going to outlaw receiving Communion, but that Mass could only be said in open space or 300 feet under the sea in open water, with all the attendees in separate individual airsuits or spacesuits. Obviously it would be physically possible (barely) for the attendees to receive Communion by unscrewing their helmets, and they might possibly even survive the experience (barely). But it would be a moral impossibility to do all that silly, murderous jumping of hoops. (So you’d be better off just holding secret Masses on dry ground on Earth, or even inside perfectly good buildings.)
So if going to Confession means that you have to walk two weeks to get there and then walk two weeks back, starving most of the way, and you also have to cross a multi-lane superhighway on foot while endangering other people and risking rape, mugging, and murder by the neighborhood on the way there, that would be a moral impossibility too.
Moral impossibilities can also involve thinking of other people’s safety, or your own. For example, you’re not obliged to go to church when you’re contagiously sick, because it would be physically possible for you but it would endanger others who are more frail than you.
For another example, it may be physically possible for you to make it to Mass a couple miles away by walking there after a really bad ice storm, but there’s also a strong possibility that you’re putting yourself in danger of injury. The time I did that, I found out abruptly that it was not quite physically possible for me to make it to church (when I suddenly fell and broke my arm in two places); so I should have had the sense to see beforehand that it was morally impossible.
It doesn’t have to be anywhere near that difficult, to be morally impossible. But there do have to be grave circumstances, or really stupid circumstances.