Organic Groups admin has can't edit roles and permissions in groups
The problem is probably that the group doesn't have it's own role/permission settings. It's using the global og roles/permissions, and so the settings within the group are all read-only.

One of the fields you can add to group entities via the OG Fields settings is a "Group Roles and Permissions" field. Once that field is added, you can edit the group entity and set the field to "Override default roles and permissions".

Once that's done, the group will have it's own permissions, and users within the group who have permission to edit the groups permissions will be able to.