When User Role Editor is activated the Schema Pro Plugin is broken in that sense that you cannot add any new schemas unfortunately. so you can go to the schema pro settings but you cannot add a new one or edit existing ones. i did proper testing on a fresh site solely with those 2 plugins so it is most certainly related to those 2 plugins unfortunately.
I suppose that URE at your site has active options related to the custom post type, like:
– Activate “Create” capability for posts/pages/custom post types,
– Force custom post types to use their own capabilities.
And current user role did not contain needed capabilities at the moment when you try add/edit schema.