You will need to place the INSERT INTO right after the CTE . So the code will be:

;WITH alias (y,z)
AS
(
    SELECT y,z FROM tableb
)
INSERT INTO tablea(a,b)
SELECT y, z 
FROM alias

See SQL Fiddle with Demo

Another way without using a CTE is by wrapping it in a subquery,

INSERT INTO tablea(a,b)
SELECT y, z 
FROM 
(
    SELECT y,z FROM tableb
) alias

Semicolon is used to terminate the statement. So when you use ;WITH, terminates the previous statement. However, that's not why you are getting the error here. The problem here is with your INSERT INTO statement, which is looking for VALUES or SELECT syntax.

INSERT INTO statement can be used in 2 ways - by providing VALUES explicitly or by providing a result set using SELECT statement.

In my case the suggested answer was unappliable, I could think it is a metter of SQL Server Version which in my case is SQL Server 2016. Alternatively you could use temp tables through this snippet of code:

;WITH alias (y,z)
AS (SELECT y,z FROM tableb)
SELECT Y,Z
INTO #TEMP_TABLE
FROM alias

Z

on db2 is possible.

Disclaimer
I will be using a complete different example for the answer purposes.

SQL script

/*
 * DBeaver Version 23.2.0.202309041200
 * `db2:11.5.8` See. https://hub.docker.com/r/ibmcom/db2/tags.
 * 
 * DDL use for the example.
 * 
 * The next three tables contain portion for the "Travel" entity.
 * 
 * CREATE TABLE "TEST-SCHEMA"."TRAVEL"  (
 * "TRAVELID" INTEGER NOT NULL, "PASSANGER_QUANTITY" INTEGER ) IN
   "USERSPACE1" ORGANIZE BY ROW;
 *
 * CREATE TABLE "TEST-SCHEMA"."TRAVEL_RATE" (
 * "TRAVELID" INTEGER NOT NULL, "RATE" DOUBLE ) IN "USERSPACE1" ORGANIZE
   BY ROW;
 *
 * CREATE TABLE "TEST-SCHEMA"."TRAVEL_TIME_TO_ARRIVE"  (
 * "TRAVELID" INTEGER NOT NULL, "TIME_TO_ARRIVE" INTEGER ) IN
   "USERSPACE1" ORGANIZE BY ROW;
 *
 * 
 * This table show a full "Travel" entity collecting data from the tables listed above.
 * 
 * CREATE TABLE "TEST-SCHEMA"."TRAVEL_RESULT"  (
          "TRAVELID" INTEGER , 
          "PASSANGER_QUANTITY" INTEGER , 
          "RATE" DOUBLE , 
          "TIME_TO_ARRIVE" INTEGER )   
         IN "USERSPACE1"  
         ORGANIZE BY ROW; 
 * */

-- This sintax use `WITH` and `INSERT` statements.

INSERT INTO "TEST-SCHEMA".TRAVEL_RESULT (
  TRAVELID, PASSANGER_QUANTITY, RATE, TIME_TO_ARRIVE
)
WITH 
collect_travel AS (
    SELECT 
        TRAVELID, PASSANGER_QUANTITY
    FROM "TEST-SCHEMA".TRAVEL
),
collect_rate AS (
    SELECT 
        TRAVELID, RATE
    FROM "TEST-SCHEMA".TRAVEL_RATE
),
collect_time_to_arrive AS (
    SELECT 
        TRAVELID, TIME_TO_ARRIVE
    FROM "TEST-SCHEMA".TRAVEL_TIME_TO_ARRIVE 
)
SELECT 
    collect_travel.TRAVELID,
    collect_travel.PASSANGER_QUANTITY,
    collect_rate.RATE,
    collect_time_to_arrive.TIME_TO_ARRIVE
  
FROM 
    collect_travel
    LEFT JOIN collect_rate ON collect_travel.TRAVELID = collect_rate.TRAVELID
    LEFT JOIN collect_time_to_arrive ON collect_travel.TRAVELID = collect_time_to_arrive.TRAVELID
;

Result set

I hope it helps you!