Can I place a sell order on a scrip when I already have a Buy position in open in the same scrip?

You cannot place an opposite order in a scrip if you already have an open position in the same scrip. For example, if you place a buy Super Multiple order with a sell Stop Loss order, then you cannot place a Sell Super Multiple order in the same scrip till the buy position is squared off, i.e., to take a sell position you have to square off your buy long position first and vice-versa.