Account generator Rule

I have created an a rule for Asset clearing account and attached it to the booking accounting template and when i tried to book contract it gives me the following error

"Transaction supplied insufficient source(s) and Account is not defined on the Accounting Template. Please specify the account to be used on the accounting template line"

i can define account but i want it to be drives automatically based on asset category

I defined account generator source as follow :

Account Generator Rule                     Source Table                   Source Column.

Asset Clearing                                   Asset categories              Asset clearing