Did you accept the mission? Did you attack the right person? Did you send the correct gun to the person? Did you send it to the correct person?
If you have multiple of the same gun, you have to send the exact one that you used in the fight. Take a note of it’s quality or accuracy/damage figures, equip it and make a damaging hit (or defeat if that’s what the missions is asking you to do) with the gun. Then unequip it and press send. Make sure you send it to the right person or you’ll have to repeat the whole process using a different gun.
I realise this missions is probably out of time now but if you come across this again let one of the Helper staff know and we can guide you through it.