@sid7039 are you sending at least 20 STR? You need to send enough to meet the minimum balance, so a little more than 20STR.
Yes, trying to transfer more than 20
Bittrex contacted me and said that there are problems with accounts with less than 20 balance that hadn't been ever used. They are looking at it.
Case closed. Problem solved. Couldn't transfer from Bittrex because my account was less than 20 balance and hadn't been ever used = inactive account. Good man helped me sending me 20 STR what activated my account and now transfers/deposits are working.