Abii, first off, as Szabo said, check with the mission giver to see if you can cancel the mission. I believe that was a viable option for missions, though I can't recall as it's been a while since I've done them.
As far as the crystals, verify two things. One: check your Rank Bar under Status -> Profile and see that it's not full. I don't know what the exact position of the Rank Bar should be, but if it's full then you shouldn't need to donate more crystals to skip a mission (assuming the mission is skippable). TWo: Make sure you're donating the crystals to the correct NPC. It's not the gate guard that takes the crystals, it's the Conquest Overseer, aka Signet guy. NPCs usually don't offer dialogue if you're trading them an item, but I'm guessing the guy is actually telling you he's not accepting them because he's expecting the crystal bass. Thus I'm thinking you're trying to give crystals to the Gate Guard instead of the Signet guy.
Hope these two things resolve your problems. If not, Seedling has the right idea by telling you to go fish up the Crystal Bass. Take a fishing rod and a Minnow to Jugner Forest and look for the Crystal Spring at J-9. It's actually a pretty pond and something you might want to see at least once. Happy questing! :o)
Pagronith - Valefor Server