Error Creating Workflow Config

I’m creating a Purchase Order Workflow and have created a new field on the PurchTable Table called TotalPOAmount. I have verified the field is populated with the total amount of all lines on the PO. When I go into the Workflow Configuration and create a new workflow I get an error when I use this variable in the Set Condition for use area on the General Tab and the Set Automatic Actions areas on the Detail Tab. On the General Tab, my “Set Condition for use” looks like:

Where ApprovalLimit.Limit >= field PurchaseOrders.TotalPOAmount

As soon as I click on the Save Icon, the error I get is:

The ‘Set condition for use’ condition could not be validated. Fields are missing, or you do not have permission to access the fields.

I have checked all of the security permissions for this field and everything looks as it should. Since it is not working, I must be doing something wrong but I don’t know what it is. Can someone give me a clue on where I should start looking?

Thanks, Jerry