Before you can cancel the event, please be aware of feedback start-end time.
Because, after feedback has been executed, you can not cancel the event, which means if you want to cancel an event, you should cancel it before feedback opening.
Step 1: Under Projects main menu, pick the project that contains an event you want to cancel
Step 2: under the Event tab, click the ellipsis (...) of the event you want to cancel
Step 3: Then, click Cancel Cvent