I've been trying to apply a new migration and I was getting the following error:
The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_dbo.Parent_dbo.Childs_ChildId". The conflict occurred in database "parents_db", table "dbo.Childs", column 'ChildId'.
In my case, the table contained data. Therefore, either you have to assign a default value, set it as nullable or delete data from the database.