Используйте «Выражение таблицы», как в:
select sum(Total_Coupon_To_Be_Used) from (
SELECT dtc.coupon_type_company_name,
count(*) * dtc.coupon_type_company_coupon_amount as 'Total_Coupon_To_Be_Used',
dtc.coupon_type_company_coupon_months_combinable
FROM [dbo].[coupon_type_Company_User] dtcu
JOIN coupon_type_Company dtc ON dtcu.coupon_type_Company_ID = dtc.id
JOIN person p ON dtcu.userID = p.userID
WHERE coupon_type_company_coupon_is_combinable = 1
or coupon_type_company_has_coupon = 1
and dtc.companyID = 1081
AND p.is_active = 1
GROUP BY
dtc.coupon_type_company_name,dtc.coupon_type_company_coupon_amount,
dtc.coupon_type_company_coupon_months_combinable
) x