Key Field Has No Option to Assign Reference Proxy

Does the child table have any lookup fields?  It has to have some lookup fields (at least 1) as candidates to be the Reference Proxy.

It has a LOT of lookup fields.

Side note - Reference Proxies are set in your relationship field between your parent/child using a lookup value per coaches comments. From your screenshot - it looks like you're looking at the key field in your table, not specifically the field that defines the relationship

 Chayce Duncan | Technical Lead
(720) 739-1406 | chayceduncan@quandarycg.com
Quandary Knowledge Base

Your relationship shows the table as a CHILD table; not the parent.  Your Trust Roles would need to be a PARENT for a Proxy Field selection to be made in the specific relationship connection field

The Key field itself will not have a Proxy field; the Related Reference field pointing to the key field will.

Great, I'm not sure how I got so mixed up, but I found it now and have created a proxy. Thank you!