SQL Oracle update with join and case missing right paranthesis

basically I don't know what is wrong with my query, can u help? : It gives me: [42000][907] ORA-00907: missing right parenthesis

UPDATE
(SELECT CAR_CODE as OLD, V.FKEY as VCODE, S.FKEY as SCODE, UA.FKEY UAKEY
 FROM CARS EU_U
          inner join DICT_CAR_CODES UA
                     On EU_U.CARCODE = UA.id
          left join DICT_CAR_CODES_S S
                    On EU_U.CARCODE_S = S.id
          left join DICT_CAR_CODES_V V
                    On EU_U.CARCODE_V = V.id
 WHERE EU_U.CAR_CODE is null
) t SET t.OLD = (CASE WHEN t.UAKEY='X' THEN (t.SCODE  )
             WHEN t.UAKEY='Y' THEN (t.VCODE  )
             WHEN t.UAKEY='Z' THEN ('ZAT' as NVARCHAR2(10))
             Else null End);

I was able to reproduce your issue, and as @Littlefoot's answer pointed out it is coming from the AS part.

I think you can use cast('ZAT' as NVARCHAR2(10)) instead.

I am not sure the data types you are using for the tables, I just created few sample tables to reproduce your issue.I also tried using MERGE and CAST and it works fine, however do check the data types as mine are just examples for testing.

Fiddle

MERGE INTO CARS EU_U
USING (
    SELECT 
        EU_U.CAR_CODE AS OLD,
        V.FKEY AS VCODE, 
        S.FKEY AS SCODE, 
        UA.FKEY AS UAKEY,
        EU_U.CARCODE  
    FROM CARS EU_U
    INNER JOIN DICT_CAR_CODES UA
        ON EU_U.CARCODE = UA.id
    LEFT JOIN DICT_CAR_CODES_S S
        ON EU_U.CARCODE_S = S.id
    LEFT JOIN DICT_CAR_CODES_V V
        ON EU_U.CARCODE_V = V.id
    WHERE EU_U.CAR_CODE IS NULL
) t
ON (EU_U.CARCODE = t.CARCODE)  
WHEN MATCHED THEN
UPDATE SET 
    EU_U.CAR_CODE = 
        CASE 
            WHEN t.UAKEY = 100 THEN t.SCODE
            WHEN t.UAKEY = 200 THEN t.VCODE
            WHEN t.UAKEY = 300 THEN cast('ZAT' as NVARCHAR2(10))
            ELSE NULL 
        END;