Are you calling a PS1 file or is the script input directly into PDQ? If the script is directly in PDQ, does it run if you reference a script file instead?

I have tried both methods.. imbedding the command directly in the Powershell step window, and created a .ps1 script in my Repository and pointed the PS step to it using the Insert PowerShell Script option. Note I have also tried " " and ' ' and "' '" around the task name to to avail. the task is on the root of the scheduler library, so no need to specify the path.

Just realized you have a typo in Update. Can you see if that's the issue?

Nope, corrected that and still the same error output:

the value of the property and retry.
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : ObjectNotFound: (MPI Update:String) [Unregister-ScheduledTask], CimJobException

I made a task called "MPI Update" and was able to run your command over Inventory and Deploy and both worked. Can you find the task over PDQ? Maybe you can get the task and pipe it into unregister.