Can't add a custom field on product list view in entity order

Dear Experts,

I m trying to add a custom field in the order view entity.

My aim is to add Product cost to order per product line item. To do that I first created a custom field that can be entered manually which I ll made obligatory to enter product cost. 

Then in the form edit I m trying to add this field on product details section 

But the custom field I created is not coming to the list to be selected.

Need help.

Regards,