Try This:
SELECT
T1.Company_accountNo As Account_no,
sum(T2.Amount) As Total_Amount,
count(T1.Company_accountNo) AS No_of_Cheque
FROM Table1 T1 Inner Join Table2 T2 ON T1.id = T2.company_accnopky
Group By T1.Company_accountNo
I Think You are Missing Group By