First, you need to return the result ordered by follower as required 'Please display the result in follower's alphabet order.'
Also there is one test case, in one column, it is 'e' and 'F' while in the other column it is 'E' and 'f' and so on. The test case want you to return the 'case-sensitive' value in the follower column... I do not think it is a good test case.
I cannot change your sql to an acceptable one, my solution is:
select t.f follower, count(distinct f.follower) num from follow f inner join (select distinct follower f from follow) t on f.followee = t.f group by t.f ORDER BY t.f
while if I change it to 'select f.followee', it will fail although in fact 'f.followee = t.f'