why require Foreign Key on insert recored into table Entity

in Student info there is primary key admission_no, that lies as foreign key on other tables. whenIi insert record in Student info a message show One or more validation errors was detected during model generation:

 

CodeFirstNamespace.admin_fee: : EntityType 'admin_fee' has no key defined. Define the key for this EntityType.

CodeFirstNamespace.month_fee: : EntityType 'month_fee' has no key defined. Define the key for this EntityType.

CodeFirstNamespace.result: : EntityType 'result' has no key defined. Define the key for this EntityType.

 

how I insert a record into Student info table