You may already have an open order on your position. You will need to cancel the first open order before placing another one.
For example, you have +100 position in an F&O contract (100 quantity bought) with an open exit order at SL, or a limit order of the same contract for 100 quantity. Now, if you try to place an exit order at market price, it might get rejected. This is because the second exit order will be treated as a new short position and this would require significantly higher margin. You can either modify your open order to market (steps mention), or you can cancel the open order and place the exit order at market again.
Steps to modify the open order