Cost or Expense Must be Entered Using an Active Timekeeper Error

The issue here is with the "Person" field.  That field needs to be populated with a timekeeper or the user "Firm."   

Person: The Billing Attorney for client advances or the attorney who incurred the costs on behalf of the client (ex. travel costs, court costs, etc.). The default is usually set to Firm but can be changed to accommodate tracking client cost advances by timekeeper.