The easiest way i can think of is to create a boolean variable inside the character bp/class and set it to true after you have talked to npc. after that make a condition which will only allow you to pickup the item if the variable is true.
so for example in blueprints: if you are using a trigger box, on the actor begin overlap check if the variable is true using a “branch” node and then do the rest of the logic using the true execution pin in the branch.