IIRC, Bittrex charges a fee of 0.025 NEO for every withdrawal, and NEO isn't divisible. Try to request a withdrawal of X.025 NEO.
Thank you for your reply.
Yes, earlier I was trying to withdraw all my funds from Bittrex which includes some divisible. First I didn't notice that warning message but later when I adjusted my withdrawal amount it worked fine.
But Bittrex support team really very bad because until now they never replied to my ticket. It makes to worry about using them in future because if we face some real issue which we can't fix then don't know how long we may need to wait.