Correlated Subquery SQL Server 2014

Add an alias for the derived table before the ; for example VendLargestUnpaidInv:

Select Sum(LargestUnpaid) from
 (Select   Max(InvoiceTotal) AS LargestUnpaid from Invoices 
  where InvoiceTotal-(PaymentTotal+CreditTotal)<0 group by vendorID ) VendLargestUnpaidInv;

Or to make it more readable, using CTE (Common Table Expression)

With VendLargestUnpaidInv as 
( 
    Select   Max(InvoiceTotal) AS LargestUnpaid from Invoices 
    where InvoiceTotal - (PaymentTotal+CreditTotal)<0 
    group by vendorID
)
select sum(LargestUnpaid) 
from VendLargestUnpaidInv;