GDAX sell order issue

[Alcam]

Would someone be able to explain to me what is the reason behind getting an error when placing a limit sell order on GDAX with the message 'insufficient funds".  This afternoon I was trying to cash out, selling BTC to EUR.  I set the limit order at just above the market rate, hit place order and then I got the message.  I was trying to sell 100% of my BTC which I thought might have been the issue so I reduced it by a bit to cover any fees but it still wouldn't go through.  I ended up having to sell BTC to my Coinbase EUR wallet and pay their exorbitant fee!!!  I'm new to GDAX and cc trading in general so I obviously did something wrong but I'd appreciate an explanation so I can avoid doing the same thing in future. Thanks in advance.

[DannyHamilton]

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).