Throughput Time between Null to First Set Delivery Block

We are trying to build a dashboard to calculate throughput time between two activities.

  1. Create SO Item with no block (’ ')
  2. First Delivery Block (ZX)
    We are using the following PQL query is returning no values. Can you please guide us whether the pql query is correct?

AVG (DAYS_BETWEEN(
PU_FIRST(“VBAP”, “_CEL_O2C_ACTIVITIES”.“EVENTTIME”, “_CEL_O2C_ACTIVITIES”.“ACTIVITY_EN” = ‘<%= start_activity_SO %>’ AND “_CEL_O2C_ACTIVITIES”.“CHANGED_FROM” = ’ '),
PU_LAST(“VBAP”, “_CEL_O2C_ACTIVITIES”.“EVENTTIME”, “_CEL_O2C_ACTIVITIES”.“ACTIVITY_EN” = ‘<%= change_dev_block %>’ AND “_CEL_O2C_ACTIVITIES”.“CHANGED_TO” = ‘ZG’)
))