If you have set the system flow rate with one or more fixed flow pumps, you will need to leave one of the system loops uncontrolled (i.e. set the loop FCV to the fully open position). The loop that you need to leave uncontrolled is the most hydraulically remote loop (the one where the control valve has the least amount of differential pressure available for control).
NOTE: If there are a lot of FCVs in your system with invalid results, a quick way to pick the one with the highest negative (-) pressure gain is to display the List Window for the controls and then sort the list by the control valve dP. The control valve with the highest negative dP is the most hydraulically remote.