Someone maybe add check logic in userexit_check_vbap. For example vbap-PS_PSP_PNR.
However, it did not work as well. Although the validation subroutine was successfully called in USEREXIT_CHECK_VBAP and error message dialog box was shown if entry was not correct, but when i clicked on the dialog box / pressed the 'enter' key, the wbs element field was not opened for re-editing (it was still shown in grey and disallowed for user entry). Just like below:
Here is the solution
add code in USEREXIT_CHECK_VBAP
DATA P_FLG TYPE C.
"Get FLAG From Share Memory.
IMPORT P_FLG FROM MEMORY ID 'ZTEST'.
IF VBAP-PS_PSP_PNR = '00002650' AND P_FLG IS INITIAL.
export P_flg = 'X' TO MEMORY ID 'ZTEST'.
MESSAGE e100(zzdea_msg).
ENDIF.
IF P_FLG = 'X'.
CLEAR VBAP-PS_PSP_PNR.
ENDIF.
FREE MEMORY ID 'ZTEST'.
when error message was shown just click Enter, WBS Elemet is cleared.
