You shouldn't use that crap exchange at all, and all other yield earning platforms that earns some magical tokens are also semi-scam ticking time-bomb getting ready to explode.
If you followed all the history of Yobit you could see many members suggested to stay away from all of their ponzi schemes, and this yofarm rewards is just one of them.
Your only hope is to try contacting their support, but good luck trying to get any real answer from them.
Never keep any Bitcoin on centralized websites that earn's you some virtual crap.