[bug: can't trade platinum. what do i do?]

hey guys so I have this problem: everytime i try to trade with someone (and i mean literally EVERY time) it happens that when i want to give the other player some platinum, the platinum counter says that i have 0 platinum, even if I actually have some (something like 18). Even if i try to set a platinum quantity the counter just doesn't move, please help, I'm pretty new so i started wanting to trade only a little time ago but now this is really a serious problem.
I would really appreciate if someone could find a solution because I'm quite desperate.