User field as relationship reference field

Hello! I am trying to construct a relationship in which the key field and the reference field are both User fields. To demonstrate: In my parent table, Workers, the key field is Worker User Name and in my child table, Goals, I want the reference field to be another user field, named Worker Name. I have a very similar relationship between the same parent table, Workers, and another child table.

I just cannot seem to set up this intended relationship between Workers and Goals. Each time I do the system skips over the step to choose a reference field and automatically generates a "Related Worker" field. 

Any common reasons why a user field can't be used as a reference? Any particular tricks I should try?

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Chyanne
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