Problem description :
Users are getting error when trying to create a new instance. User tried to create new instances but we are able to create only work orders, not the actual documents which will create the workorders.The screenshot of error message is attached below.
Problem Solution :
Usually, above error occurs when "Approval Work Task" is enabled for that Document class but Task template is not specified from drop down menu.
Make sure that Document Class that you are using under extended behavior has that template selected.
Refer below screenshot -
Hope this helps.