I don't think that is the right field. If you click the "Field Check" you will see that field corresponds to a 10-char field called Group Feature. Even if you change that field to mandatory, the Grouping field in transaction BP still does not show as mandatory.
If you want to change the Grouping field to mandatory, you can do it in GenIL model (transaction GenIL_Model_browser) but enhancing the existing enhancement BUPA_CUSP. It can enhanced either directly in the GenIL model field properties or in the GenIL model handler class (you need to create a custom handler).