You need to understand the difference between a trade/signal that is "working" (i.e. has not been filled yet) and a position.
You can cancel a signal that is working.
You can close a position.
You cannot "cancel" a position.
If you have no "working" signals - there is nothing to cancel.
If you have Open Positions - those you can close or replace.
Note that when you enter a signal that has not yet filled (i.e. it is "working") it will appear in your Recent Signals table under the Trade Record of your strategy. It will also appear in your Trade Blotter and you will be able to Cancel it. (You'll see a "cancel" link beside each working order.)