Tasks cannot be cancelled when using the task console on the vSphere Client.
The same tasks can be cancelled using the recent task bar on the vSphere Client or when using PowerCLI
vCenter Server Appliance
This is a known issue and will be fixed in a later release.
Use the recent tasks bar to cancel tasks or use PowerCLI.
To use PowerCLI to cancel a task :
Connect to the vCenter Server with PowerCLI and run the following commands:
Get-Task -Status "Running" | select Name, Id, State
Get-Task -Id "Task-ID-Here" | Stop-Task -Confirm:$false