Please take a look at postion control screen and Pur Req below.

I expect that cost center value is mandtory on pur req screen.

 
Control type C58 may require cost center value with Mandatory.
As Pur Req registered,  pre posting sub screen is pop-uped when saving.
However, saving transaction without cost cent value could be allowed.
After saving, pre posting pop up screen no longer is shownd again.

Mandatory rule doesn’t work when Pur req resister process.
Is any  setup insufficient?