Step 1 of installing a helicoil requires running a tap into the stripped hole and probably getting metal chips inside the caliper like gmtech1 said. Unless you can prove it was previously damaged you'll need to buy another new caliper,bolt and any metal sealing gaskets for the bolt. Chalk it up to experience and move on.
Sometimes it helps to mount the caliper loosely so it can be moved around some then start that banjo bolt though the line and thread it into the caliper using new sealing gaskets until lightly seated. If it binds or gets hard to turn, stop, somethings wrong. Take it off and examine it. Once its seated properly tighten up the caliper mounting bolts,torque 'em to spec, then the line to spec. Then go through the bleeding process. Then check for leaks. HTH