Im trying to round robin to a user lookup field on an opportunity. The details of the fields are below and I have tried everything I can think of.
Error: An Apex error occurred: System.SObjectException: Invalid field...
Process Builder

Field Details:

Tried:
Field Reference: API Name, Field Name>User ID, Field Name>SF User Id, Field Name>Username
String: API Name, Field Name, Field Label, Field ID
Version:
