are u sure it's 100% lined up ? Spray abut of degreaser around the nut and wipe it to clean any excess brake fluid off too
Try loosening off the brake booster from the back ( pedal side) to give some play and easier to line up the lines - also remember to use your fingers to try and get it to bite before using a flare nut wrench to tighten , you can easily round the edges so be careful