I can log in to the server using a domain account without administrator privileges and create a task in the Task Scheduler. I am allowed to do an initial save of the task but unable to modify it with the same user account. When changes are complete, a message box prompts for the user password (same domain user I logged in with), then fails with the following message.

Task Scheduler cannot apply your changes.
The user account is unknown, the password is incorrect, 
or the account does not have permission to modify the task.

When I check Log on as Batch Job Properties (found this from the Help documentation):

This policy is accessible by opening the Control Panel, 
Administrative Tools, and then Local Security Policy. 
In the Local Security Policy window, click Local Policy, 
User Rights Assignment, and then Logon as batch job. 

Everything is grayed out, so I can't add a user. How can I add a user?