there is a post about this here: Affiliation mandatory, OJS 3.0.1.0 - #10 by primozs
And, what code modifications would be necessary. Note that the code that would need to be modified would likely be different as this uses an older version of OJS (you didn’t indicate your version of OJS, but I’m going to assume that you might be using a more recent version like 3.3 or 3.4).
-Roger
PKP Team