Unable to put Warning messages in ME21N and ME22N in Exit / BAdI in ECC 6.0

Dear All,

I want to display a warning message when saving Purchase Order through transactions ME21, ME22, ME21N, ME22N.

I tried user exit, EXIT_SAPMM06E_012 and BAdI, ME_PURCHDOC_POSTED.

But the warning message could not be displayed.

I am working on SAP ECC 6.0.

Please help.

Regards,

Satyendra