I also suspect the nut is cross threaded. It also looks dirty, and appears that it was wrenched with pliers? Spray it with brake kleen, then inspect it carefully as mentioned before, and make sure the nut isn't cross threaded. Use a righteous wrench on it. It's a specialty flare connection, so it should spin pretty free right up to where it's tightening the flare. BTW, that's a steel injector line. If the nut is boogered up, you may have to get an OEM replacement line assembly. Good luck with the repair.
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