sum(case when TA011='1' then 1 else 0 end ) and
sum(case when TA011='2' then 1 else 0 end )
is part of the expression. Only the complete expression can use aliases, and part of the expression must not use aliases.
select sum(case when TA011='1' then 1 else 0 end ) /sum(case when TA011='2' then 1 else 0 end) as alias
from MOCTA where TA003 between '20081201' and '20081231' and TA001 = '5120'