From what I can see on that screenshot, the element you have attached to a hierarchy, meaning that actor will pick up the setting from the group. And if said group is not spatially loaded, the asset’s checkbox will be disabled and greyed out. This could also happen in the main landscape actor.
Another reason could be that Streaming has been disabled in this level, so please make sure it’s enabled in world settings. You can also check the related threads below for further reference: