Payment Processing Tab is disabled in ME21N screen

Hi,

I have a client requirement in which the Retention values should be entered in Purchase order.

I need to make Payment processing tab appear in the Purchase order screen wherein Retention values can be entered. But as a default it is not appearing in ME21N.

I tried to make "Retention" field in Field selection key as mandatory and assigned it to Purchase order document type. But still I do not find that tab in ME21N.

Please help.

Thanks.