CREATE TABLE TBL_MKB10 (
    ID          INTEGER NOT NULL,
    PARENT_ID   INTEGER,
    MKB_VALUES  VARCHAR(300)
);

ALTER TABLE TBL_MKB10 ADD CONSTRAINT PK_MKB10 PRIMARY KEY (ID);
CREATE INDEX PARENT_IDX ON TBL_MKB10 (PARENT_ID);

CREATE OR ALTER PROCEDURE GET_PARENT (
    ID TYPE OF COLUMN TBL_MKB10.ID)
RETURNS (
    PATH TYPE OF COLUMN TBL_MKB10.MKB_VALUES,
    PARENT_ID TYPE OF COLUMN TBL_MKB10.PARENT_ID)
AS
BEGIN
  WHILE (:ID > 0) DO
  BEGIN
    FOR SELECT M.PARENT_ID, M.MKB_VALUES
        FROM TBL_MKB10 M
        WHERE M.ID = :ID
        INTO :PARENT_ID, :PATH; --вероятно, затык тут
        ID = :PARENT_ID;
        SUSPEND;
      END
END

Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 13, column 31.
;.

CREATE OR ALTER PROCEDURE CREATE_DOCSETTINGS_NEW_USER (
    NAME TYPE OF COLUMN RDB$DOCSETTINGS.NAME,
    NOTE TYPE OF COLUMN RDB$DOCSETTINGS.NOTE)
AS
DECLARE VARIABLE ROOT_ID INTEGER;
DECLARE FUNCTION GET_DOCSETTINGS_NEW_ID 
RETURNS INTEGER 
AS 
BEGIN 
   RETURN GEN_ID(GEN_RDB$DOCSETTINGS_ID, 1);
END

BEGIN
  ROOT_ID = GET_DOCSETTINGS_NEW_ID; -- получаем корневой ID
--<skiped>
END;

Column does not belong to referenced table.
unsuccessful metadata update.
CREATE OR ALTER PROCEDURE CREATE_DOCSETTINGS_NEW_USER failed.
Dynamic SQL Error.
SQL error code = -206.
Column unknown.
GET_DOCSETTINGS_NEW_ID.