It may appear that you're holding enough currency to perform a particular trade, but your order is being rejected. There are two possible reasons for this:
1. You haven't transferred the required level of funds to your trading account. Please check your balance on the account page. If your trading balance equals zero, it wouldn't be possible to trade, even if your main balance isn't empty. You will then have to initiate a transfer from main account to trading account by hitting the arrow in the 'Transfer' section.
2. You don't hold the required funds to pay the trading fee. Imagine you're holding 1 ETH and you're trying to buy certain tokens amounting 1 ETH. 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.
3. We also recommend to lower up to 10% the amount of coins you want to buy while setting market orders. By default the system is setup to account for fluctuations which might happen in a matter of seconds and system reserves some extra amount even though it won't be utilised. Please see more info for this case scenario here.