您的分组依据在子查询之外，因此 c.hrmort 不存在。而是分组方式 x.MrtCategory 并对子查询外的金额求和：

Select x.MrtCategory,
       sum(x.amount) as Amount
from (
    select
        case 
          when c.hrmort='CMHC' then 'CMHC'
          when c.hrmort='Genworth' then 'Genworth'
          when c.hrmort=''
            and a.purp in ('P16','P17') 
            and c.tenure in ('Freehold','Condo','Strata')
            and a.secval<1000000
            and a.amorty<=25
            and a.class in ('Standard','Stf Benefit Rate','Stf Member Rate')
            and a.totltov<80
            then 'Conventional Insurable'
          when c.hrmort IS NULL then 'Other'
          else 'Conventional UnInsurable'
        end as MrtCategory,
--        sum(a.amount) as 'Amount'
        a.amount
    from      ODS_WB.dbo.lnap as a
    left join ODS_WB.dbo.cust as b on a.no_=b.no_ and a.surname=b.surname
    left join ODS_WB.dbo.scur as c on b.rowno=c.rowno_custscur_cust and a.secval=c.secvalue and c.status='active'
    where year(a.appdate)=2020
    and   month(a.appdate)=6
    and   a.apptype='Mortgage'
    and   a.sourcecode in ('FI',' ')
) as x
--group by c.hrmort -- c doesn't exist outside of the above subquery
group by x.MrtCategory