What is Bittrex?
Bittrex is a crypto-to-crypto exchange. You can only trade with cryptocurrencies on a crypto-to-crypto trading exchange. In other words, fiat currencies cannot be used.
Note: Fiat currency is a state-sponsored currency. Examples of fiat currencies are the US Dollar, British Pound, and Chinese Yuan.
However, you can invest using USDT. US Dollar Tether is a cryptocurrency whose value is tied to the US Dollar. 1USDT is always worth 1USD. Bittrex has more than 190 different cryptocurrencies. The site also features 284 unique trading pairs. The exchange rate is formed between two currencies between the trading pair.