Why can't I add a Take Profit or Stop loss to my order in TradingView?

Interactive Brokers only allows orders to be bracketed so long as the order is still open and has not been executed. Once an order has executed, no modifications or brackets can be added to an order.

Child orders can be added to open orders on the TradingView “Trade” window, or by selecting the “TP” or “SL” icons beside your order in the chart.