How do I solve the error"'table' contains no primary or candidate keys that match the referencing column list in the foreign key."?

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C#

MySQL

MVVM

Hello everyone,

I have a problem with my database.It worked until today when I tried to build it and it said that my db relationship is not good"SQL71516: The referenced table '[dbo].[Courses]' contains no primary or candidate keys that match the referencing column list in the foreign key. If the referenced column is a computed column, it should be persisted" and also "Error 159: EntityType 'DatabaseStudentsModel.Login' has no key defined. Define the key for this EntityType"In the second case,I already have the keys defined...These are my tables:

CREATE TABLE [dbo].[RegisterTeacher] (
    [SNTeacher]  INT            NOT NULL,
    [UserName]   NVARCHAR (30)  NOT NULL,
    [pwd]        INT            NOT NULL,
    [fullName]   NVARCHAR (MAX) NOT NULL,
    [courseID] INT NOT NULL,
    [education]  NVARCHAR (50)  NULL,
    PRIMARY KEY CLUSTERED ([SNTeacher]), 
    CONSTRAINT [FK_RegisterTeacher_ToTable] FOREIGN KEY ([courseID]) REFERENCES [Courses]([courseID])
);

CREATE TABLE [dbo].[Login] (
    [SNTeacher] INT           NULL,
    [UserName]  NVARCHAR (10) NOT NULL,
    [pwd]       INT           NOT NULL,
    PRIMARY KEY CLUSTERED ([pwd]),
    CONSTRAINT [FK_Login_ToTable] FOREIGN KEY ([SNTeacher]) REFERENCES [dbo].[RegisterTeacher] ([SNTeacher])
);

CREATE TABLE [dbo].[Courses] (
    [courseID]   INT            NOT NULL,
    [courseName] NVARCHAR (MAX) NOT NULL,
    [education]  NVARCHAR (50)  NOT NULL,
    [SNTeacher]  INT            NOT NULL,
    [ClassID]    NVARCHAR (18)  NULL,
    PRIMARY KEY CLUSTERED ([courseID] ASC),
    CONSTRAINT [FK_Courses_ToTable] FOREIGN KEY ([SNTeacher]) REFERENCES [dbo].[RegisterTeacher] ([SNTeacher])
);

CREATE TABLE [dbo].[ChildCourse] (
    [courseID] INT           NOT NULL,
    [UserName] NVARCHAR (10) NOT NULL,
    [sNr]      INT           NOT NULL,
    PRIMARY KEY CLUSTERED ([courseID] ASC), 
    CONSTRAINT [FK_ChildCourse_ToTable] FOREIGN KEY ([sNr]) REFERENCES [Courses]([SNTeacher])
);

What should I do in order to get rid of the error and what did I do wrong when it came about establishing the relations between the tables?Thank you in advance!

What I have tried:

I have tried to change the PK and FK for Courses,Login and RegisterTeacher,but the database won't allow me to do anything,even if I change sth,the table won't alter.