That's what I was trying to find out, if it's just the nut part on the hard line (the part in the pic below where the flair nut wrench is on) you might still be able to get it tight enough, if the mating surfaces are smooth and un marred. The problem is the nut material is soft, you don't want to squish it. Even if it's rounded you may be able to get a grip on it with some vice grips and get it tight enough to stop it leaking and get it to a garage to have it fixed properly.
Still I have to say here, is no "compression" fitting inside the flexible (rubber or SS) line part, however the flair nut on the hard line screws into the flexible part and the flare in the hard line mates w/ the conical surface face of the fitting in the flex line's end. Compression fittings don't have flairs on them. They have cut off ends you slide on a nut then a sleeve (aka gland) and screw that into the female part of the fitting. The gland gets compressed when you tighten the nut squeezing the line you are affixing. That type you can cut off just behind the gland and reuse w/ a new gland. A flair nut needs a flaring tool but no gland. I think though like I said a normal flair tool won't work. I think the flair on the end of a brake line isn't the garden variety type. It's double folded back I think but I'm not positive.
My dad was a master plumber and I use to do a lot of pipe fitting. I have flaring tools but never used them on brake lines so I can't be sure how those are done, I just remember reading somewhere you can't just use a standard flair tool.
Of course if these aren't the proper SS brake lines and the flair surface dosen't properly mate to the flex lines all bets are off