Los 10 mejores jugadores con el promedio de bateo más alto

porque de su subconsulta (resultado de la unión) solo devuelve la columna "Batman_". tienes que devolver toda la columna que necesitas :

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;