What is "SQL error code = -206" when adding a trigger?

I am trying to add the following trigger:

SET TERM !! ;
CREATE TRIGGER reg_ral_list for Registration
ACTIVE AFTER INSERT AS
BEGIN
  FOR SELECT app_id FROM application into :appid
  DO
    BEGIN
      INSERT INTO Registration_Application_List (user_id, app_id)
values (new.user_id, :appid);
    END
END!!
SET TERM ; !!

If a user_id is added to table Registration then this user_id and all
the app_id are added to the table Registration_Application_List.

When I try and add this trigger (using the Interactive SQL) I get the
following error:

Dynamic SQL Error
SQL error code = -206
Column unknown
Statement: CREATE TRIGGER RegistrationAfterInsertUserID .....

I do not understand the "column unknown" bit. Any help would be much
appreciated.

Cheers