Virginia,
This type of Question should be asked in the Customization section of the forum to reach out to the appropriate audience. As for your issue, you cannot do what you are trying to do because you are probably running the rule on a work item and trying to pull data from a timesheet item type.
What is your process for Time? If you are going to put time into Timesheets, be sure to specify Update Actual Effort from Timesheet'. Then you want to specify the type of Approval process you are going to have. Do you want total time to include Pending (non-Approved time)?
Please provide me a screen capture or details of the Custom Field you are trying to create specifying all settings.
Sincerely,
Boris