Substitution XBLNR to ZUONR

Hi All,

Need your urgent help in below. i have written below substitution to update the GL field of ZUONR.

BKPF-BUKRS = '1000' AND BSEG-KOART = 'K' AND

BKPF-BLART = 'ZP' AND BSEG-HKONT = '220511'

Assignment Field BKPF-XBLNR

Do not why substitution is not working please help.

Please help.