Got the answer :
The system has detected that the phone number you have entered has been used on another account. If you have not used this number on another account please let us know immediately.
Bittrex does allow multiple accounts. In order to use multiple accounts you will need to verify each account to the Enhanced Level, you can do that here:
Once verified please provide all account emails you have registered at Bittrex for our records. You can then attach the Proof of Identity listed below and we will process your ticket for an account re-enable.
Required Proof of Identity
Please provide a government identification and a selfie of you holding the identifications where we can match your face. Please also write Bittrex and today's date on a piece of paper and hold it in the picture. Please make sure the text on your ID is readable in all photos. Note: This means we need all 3 things in 1 photo. You, your ID, and the paper with Bittrex and the date on it. Also we do not accept .pdf files, or links to files.