Once you loot a certain number of individual item stacks (mobs, chests, quest rewards, or cube slots filled), the server temporarily blocks you from adding new items to your inventory to prevent automated farming scripts from running 24/7.
This is the hard pill. You cannot get the Best in Slot (BiS) item right now. Accept it. Look at the second-best. Look at the third-best. Is there a level 50 blue item that has slightly less stats than the level 55 gold item? Use that. Aion punishes perfectionism. The game is balanced around the average player, not the min-maxer. Slap on the "good enough" gear, go into the Abyss, and try to have fun. The BiS item will come when the next content patch drops and makes the old gear obsolete anyway. aion you are unable to obtain items at the current time best