Error in Goods Recipt Purchase Order

Hi,

I am in t.code: MIGO_GR-- 3rd party sales order receive goods.

I was trying to do Migo_gr, the OK check box became inactive and while saving i am getting the following message: "you have not flagged any items as OK".

please advice me to slove this issue.

what is the reason for becoming greyed out the OK check box.

Thanks

ANGKS