I set the limit order at just above the market rate, hit place order and then I got the message.
You need to fill out BOTH fields.
In the "
Limit Price" field you choose the price that you want. (In your case, "just above the market rate")
In the "
Amount" field you indicate the quantity of bitcoins that you want to sell.
The quantity of bitcoins in the "Amount" field must not exceed the total amount of bitcoins that you have in your GDAX wallet (if the bitcoins are in your Coinbase wallet, then you need to transfer them to GDAX before you can place the limit order).