It could be because you are running "Media Center 27.exe" instead of "MC27.exe".
It could be because you are running a Once Off instance of the task, instead of a recurring task. I assume the time you have set is in the future, so the Once Off should work, but I haven't tried that.
I've not tried running the "Library Backup" task rather than using a "Command Line" task to achieve the same result.
Either task should show a countdown to the time it will run under the Status column in the Scheduler, as per my image. Does yours show a countdown?
Okay, I just ran a Once Off Library Backup task, and it worked fine, reporting "Finished" in the Status column once it was done. Once I set up and saved the task I could watch the countdown to execution. I checked and the backup file was there.
So I guess what is happening on the Scheduler pane for the task will be a hint as to what is happening, or not, for you.