Can't reference an aggregate field in the same query it is calculated. Try:
Code:
SELECT tLoans.LoanNumber, tLoans.AmountOwed, Sum(tPayments.PaymentAmount) AS SumOfPaymentAmount, [AmountOwed]-Sum([tPayments].[PaymentAmount]) AS RemainingBalance, [FirstName] & " " & [LastName] AS FullName
FROM (tCustomers INNER JOIN tLoans ON tCustomers.CustomerID = tLoans.CustomerID) INNER JOIN tPayments ON tLoans.LoanNumber = tPayments.LoanNumber
GROUP BY tLoans.LoanNumber, tLoans.AmountOwed, [FirstName] & " " & [LastName]
HAVING ((([AmountOwed]-Sum([tPayments].[PaymentAmount]))>0))
ORDER BY [FirstName] & " " & [LastName];
Why is PaymentAmount field in multiple tables?