This is the function I use to insert a new user into the database:
public function store_user(Request $request){
    $user = new User();      
    $user->id_user = "999/11"; // this is the problem      
    $user->name= $request->input('user_name');
    $user->last_name= $request->input('last_name');
    $user->save();
    return redirect('/users');
}
Every time I got this error
Yajra\Pdo\Oci8\Exceptions\Oci8Exception Error Code : 1722 Error Message : ORA-01722: invalid number Position : 100 Statement : insert into "users" ("ID_USER", "UPDATED_AT", "CREATED_AT") values (:p0, :p1, :p2) returning "ID_USER" into :p3 Bindings : [999/11,2020-10-27 10:18:03,2020-10-27 10:18:03,0]
 
     
    