打率が最も高いトップ10選手

サブクエリ（unionの結果）からは「Batsman_」列のみが返されるためです。必要なすべての列を再調整する必要があります：

select Batsman_, sum(batsman_runs)/count(player_dismissed) as Average 
from
(
 (select batsman as Batsman_,batsman_runs,player_dismissed from IPL_BALL_BY_BALL) 
 union all 
 (select non_striker as Batsman_,batsman_runs,player_dismissed from IPL_BALL_BY_BALL)
)
group by Batsman_ 
order by Average desc;