PR fileld mandatory while creating P.O

Hi Gurus,

I want to make PR field as a mandatory to create NB type P.O

We have assigned field selection key NBF to NB p.o type

In NBF field selection key, i ticked PR as a required entry in Reference data,item selection group.

But still while creating NB p.o,system allows to save p.o without entering the PR.

Can anybody guide is there any other settings to be made?

Regards

Mohan