you can try following code:
PU_FIRST("EKPO", PROCESS_ORDER("_CEL_P2P_ACTIVITIES"."ACTIVITY_EN") ,"_CEL_P2P_ACTIVITIES"."ACTIVITY_EN" = 'Release PO')
For the rework calculation, you can use the function ACTIVATION_COUNT(), to only consider rework activities that happened after ‘Release PO’ you would use the PROCESS_ORDER() function which gives each activity within a case an increasing number which you can then use to just take the numbers that are higher than the number for ‘Release PO’ activity.
Since this formula now gives the rework count for every time an activity happened within one case, you only want to take the maximum (=final count) of the activity. You do that using the MAX() function.
Hope this helps.