As suggested by legends above below are the only requirements
Capacities tab: Other formula: SAP008
Scheduling tab: Other formula: SAP004
Costing tab Key SAP008 along with Activity Type & cost centre etc...
you need to maintain to get the Requirement Assignment Formula to be viewed
Garry hope the issue is resolved !!!
I got the catch you must check the external scheduling is unchecked in customization along with above all I got such kind of problem.