cancel or delete work task

I wanna cancel a work assignment inside a work task as I am unable to close it, but when I hit cancel, it asks for cancellation cause. The only place where I could find cancellation cause is below, but it does not reflect in assignment screen

so please tell how to cancel a work assignment.. When I am trying to cancel the work task, it shows the same screen asking for cancellation cause but no cause is autopopulated in dropdown

so basically i want to know how can I delete or cancel a work task?