ApproxSampleSize requires a summary (dimension) table.
SELECT * FROM sample (
ON (SELECT stratum, count(*) AS stratum_count FROM
score_category GROUP BY stratum) AS summary DIMENSION
ON score_category PARTITION BY ANY
StratumColumn ('stratum')
Strata ('excellent', 'fair', 'very good')
ApproxSampleSize ('5', '10', '5')
Seed ('2')
) ORDER BY stratum, id;