The Classic Workflow allows you to define valid Statuses for each Workflow Task. It also allows an action to be defined that takes place when a Workflow Task Status changes.
This is done against the Category, Behavior Template and "Transitions" and is available for each Workflow Status:
IMPORTANT: These Transition conditions apply to the Statuses prior to the one you wish to impact.
For example, if you wish to do your conditional check at the time the "Status = Complete," you need to set the transitions against "Pending" and any other Status that may be prior to "Complete."
You won't need to put the conditions on "Cancelled" or a Status which does not allow Complete to be reached.
How to stop the Status of a Classic Workflow Task moving to "Complete" if the Change Order's Assignee is empty?
Using Classic Workflow (not CA PAM) in Service Desk Manager 12.6 and all versions above.
See the attached document which contains screenshots which guides through each step of setting up the above scenario.
This document is written for SDM 12.6, but is applicable to all higher versions at the time of writing.
If CA PAM is available, then that is an alternative way to achieve this end, but is not documented here. CA PAM allows greater flexibility.
The advantages of the Classic Workflow is that a simpler procedure is available within the CA SDM product with a lower learning level.
ITSM 14.1 Wiki: Define Behavior Template
CA Communities: Discussion thread here.