Sage 100

I'm a new user of Sage 100 ERP 2013 (v 5.00.8.0) and am trying to post cash receipts. I have a client using three checks to pay one invoice. I entered the first check fine, but when I try to enter the second check and apply it to the same invoice it says…

Hi we are running MAS90 V4.5.4.0, I get a excel spreadsheet from customer, they supply PO#, check description, doc date, gross amt, disc amt, net amount paid. Currently someone manually enters into cash receipts, we have 1000s of entrys, the customer…