Master data adds connectivity as a unique limitation - iOS

Question

I am developing an application with basic data. There are many objects that use relationships, and in many of them they are the primary key.
I am trying to use the unique constraint function in xcdatamodel, but there is no way to compile the project.
The error that Xcode shows is:
cdtool cannot compile .

This only happens with relationships, if I add an attribute in the constraints, there is no problem.

I would like to know that I am doing something wrong or that communication cannot be a unique limitation.

EDIT 1: