I'm trying to add a related list of Branch Units to the Locations page and it is not one of the options in the dropdown
Good catch, and you are right — that Location lookup does exist on Branch Unit, so let me correct my earlier framing. The reason it still gives you no related list is that the lookup is a standard/system relationship, and Salesforce does not expose a reciprocal related list on the standard Location object for it. For certain standard lookups (especially into system objects like Location), the add-this-related-list-to-the-parent-layout option simply is not offered — so you can see the lookup on Branch Unit but cannot surface it as a related list on Location.
Two ways to get the view you want:
1. Quickest: a report on Branch Units grouped or filtered by their Location field — an instant list of all branch units per location.
2. Native related list: add your own custom lookup from Branch Unit to Location. A custom lookup does generate an addable related list, which you can then drop onto the Location page layout.
So the standard lookup is real, it is just reverse-related-list blocked; a custom lookup or a report is how you surface it.
