That means that the Requestor object is not a generalization of the user object. If the requestor is an actual user of the system you should let it inherit from the user object. See documentation here: https://world.mendix.com/display/public/bestpractices/Inheritance+vs.+1-1+association
The Requestor attribute is a String now, but it should be an association to the User entity. Or, like Ronald explained, a generalization of it.