This error will occur when the amount you are trying to buy/sell is lower than the minimum buy amount. The minimum buy amount needs to be 0.001. In your config settings, (Percentage buy amount * maximum btc amount allocated) needs to be at least 0.001. So for example if you have 5% of 1BTC = 0.05, the hopper would be able to place orders because the amount is higher than 0.001. But if you would have 1% of 0.09BTC = 0.0009, the hopper would get this error.