If you haven't transferred the necessary amount of funds to your trading account, you may likely get an insufficient funds error, ensure to check your balance on the Account page. If your trading balance equals zero, you will not be able to trade.
If your trading account is equal to the exact amount of a particular token worth you want to buy, you will be unable to proceed because you will actually need to pay out that amount plus the trading fee. Try to indicate a smaller amount, so as to have enough to cover the transaction fees.