Te falta el END
en su CASE
declaraciones:
SELECT
surname,
givenname,
(CASE WHEN surname = 'Smith' THEN 2 ELSE 0 END) -- < add END
+ (CASE WHEN givenname = 'John' THEN 1 ELSE 0 END) -- < add END
AS score
FROM person
WHERE (surname = 'Smith' AND givenname LIKE 'J%')
OR (surname LIKE 'S%' AND givenname = 'John')
OR (surname = 'John' AND givenname LIKE 'S%')
OR (surname LIKE 'J%' AND givenname = 'Smith')
ORDER BY
score DESC,
surname ASC,
givenname ASC;