SELECT "Users"."UserId", "Users"."Email", AVG(COALESCE("Ratings"."Evaluation", 0)) As AvgRating
FROM "Users"
LEFT JOIN "Contents"
ON "Users"."UserId" = "Contents"."UserId" -- там нет поля UserId, есть AuthorId
LEFT JOIN "Ratings"
ON "Ratings"."ContentId" = "Contents"."ContentId"
GROUP BY "Users"."UserId"

SELECT U."UserId", U."Email", CR."AvgRating"
FROM "Users" U
LEFT JOIN (
  SELECT C."AuthorId", AVG(R."Evaluation") as "AvgRating"
  from "Contents" C 
  join "Ratings" R ON R."UserId" = C."AuthorId" and R."ContentId" = C."ContentId"
  GROUP BY C."AuthorId"
) CR on CR."AuthorId" = U."UserId"
ORDER BY CR."AvgRating" DESC