If you are holding enough of a currency to perform a particular trade, but your order is rejected. there are two possible reasons for this:
You haven't transferred the necessary amount of funds to your trading account. Please check your balance on the Account page. If your trading balance equals zero, you won't be able to trade, even if your main balance isn't empty. Kindly initiate a transfer from main account to trading account by hitting the arrow in the "Transfer" section.
No funds would be left to pay the trading fee. Imagine you're holding 1 ETH and you're trying to buy certain tokens with that whole amount. It means that you'd need to pay 1 ETH + trading fee (0.01 ETH), which is more than you're holding at the moment. Try to specify a lower amount of ETH to be spent.
Whenever you are trading, make sure to have at least an additional 10% of the amount of your market order deposited in your account.
This means, if you place an order for 1 BCH, make sure to have 1 BCH (worth of whatever you are buying with) and an additional 0,1 BCH in your account.