Hi Akram,
Are you trying to Group on the PO field and add Line, GRN Cost and Pay Amount as Summaries at the PO Group?
IF yes, go to the Group Expert > Add the PO field to the Group List. This should create the group.
Next, go to insert summart > Choose Line Total field > Choose Sum > Under 'Summary Location' choose Group #1.
Do this for the other two fields as well.
-Abhilash