Please help

Hi all,
Can someone find my error? I have add a new column in my existing project which is a Foreign key from a relationship
I have a table Stoixeia which i like to add the foregn key  id_user from the table _user

I use the following script but it faild with an error.

   sqlexecute('ALTER TABLE Stoixeia ADD FOREIGN KEY (id__user) REFERENCES _user (ID)');