Hi Prem,
1) Change BKPF-TCODE for SYST-TCODE and create a Set to replace 'F-28' , you may need other tx as well (ie: FB05),
2) add to your rules BSEG-KOART = 'S'
3) In OB28 step onto the Validation (not the step) and go to Details -> Trace on . Post your F-28 and check your validation rule
Kind Regards