The validation rule should be on the Case object.
The validation rule should be named 'Mark_as_Escalated'.
The validation rule should fire if someone tries to set a case as escalated and it is closed, closed when created, or does not have a priority of High.
The validation rule should display the error message 'You can only set a case as escalated if it is high priority and not closed' under the Escalated field when triggered.
Add the 'Escalated' field to the Case page layout.

I continue getting an error on the Workflow Rule in the Trailhead excercise that says a workflow field update action was not found, but I have created it.  

Challenge not yet complete... here's what's wrong: 
The 'Set Case to Escalated' Workflow field update action was not found.
Create a workflow rule that will update a field.In some instances, a case must automatically escalate. Create a workflow rule that will set the escalated flag and send a task to the case owner, if a case is not closed and its priority is set to High.

Can someone please help me with this? What is the correct formula for the workflow itself? Perhaps the error is in my current formula.
Thanks!