I'm reading the documentation and looks like you are using the wrong format.
e.g:
Here is the correct one for BTC/ETH:
https://novaexchange.com/remote/v2/market/info/BTC_ETH/Edit: just saw you want all BTC markets. In this case I believe is a problem in their API.
https://novaexchange.com/remote/v2/markets/ is working but it's showing all currencies and not only BTC.