i believe you are getting the error because in your query, itCount is an alias for the subquery, i.e. derived table
could be wrong; wouldn't be the first time
the GROUP BY seems unnecessary in that construction, too
luckily your query can be rewritten in a more straightforward manner
you'll want to sort descending, too
Code:
select top 10
sFName as sTemp
, count(*) as itCount
from tblData
where sSurname like 'H*'
group
by sFName
order
by itCount desc