...I open the order and the only option I have is to "Cancel Order."
I'm assuming it's because the order has not yet been filled.
Once the order is filled I will be able to close the trade...
Which happened because you put the purchase price below the market price, to which the BTC price has not decreased. In this case, there is no specific deadline when your purchase order will be executed. Obviously, instead of a limit order, you had to make a purchase at the market price.