I'm trying to sell my SUP but Bittrex keeps redirecting me to dogecoin.
That's strange, I just checked my bittrex account and its working fine when i click on SUP it takes me to the SUP orders screen like normal. Try to login to your bittrex account and use this link
https://bittrex.com/Market/Index?MarketName=BTC-SUPIts fixed now. It was when I clicked the SELL button.
It has happened before, like, last year.