When you use CASE statments you must return like data types. You need to change the "ELSE 0" to a "ELSE '0'" or "Null". However if you change you approach just a bit I think you can achive your goal...

Try
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SELECT skill_Id,
sum(CASE final_rating WHEN 5 THEN 1 ELSE 0 END) AS "5",
sum(CASE final_rating WHEN 4 THEN 1 ELSE 0 END) AS "4",
sum(CASE final_rating WHEN 3 THEN 1 ELSE 0 END) AS "3",
sum(CASE final_rating WHEN 2 THEN 1 ELSE 0 END) AS "2"
FROM viewfinalrating
GROUP BY skill_id
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