protože z vašeho poddotazu (výsledek sjednocení) vracíte pouze sloupec "Batsman_". musíte vrátit všechny sloupce, které potřebujete:
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;