FK relationships between schemas in azure SQL don't work
The error message is self-explanatory. You need a primary key or unique index/constraint on the UserGUID column in dbo.[User].

ALTER TABLE dbo.[User] 
  ADD CONSTRAINT PK_User
  PRIMARY KEY (UserGUID);


If that is there (you edited it in later), you shouldn't get that error message.

If you are only getting this error message when deploying a dacpac, can you post that dacpac somewhere? I can't make that code fail on Azure SQL Database by running it directly, but it's possible the dacpac has creation scripts listed in the wrong order, or the constraints are separated out into a different (later) step and aren't in the right order either, or the dacpac is only deploying changes to one schema and the other one hasn't been updated.