Our system can only auto-generate a PayPal Mass Payments file for the last completed month, from your Seller Admin > Pay Your Affiliates screen.
If you need to review affiliate earnings going further back, you can go into your Seller Admin > View/Download Transaction Log, select a date range and a specific affiliate, then click View Log to view data for only those sales. Then if you wish, you can click Download Log to save that log view to a file on your computer, which you can unzip and open in any spreadsheet program such as Excel.
You could also just select a date range alone (without choosing a specific affiliate) and then View Log, then click Download Log to save that log view to a spreadsheet file where you can sort by the Affiliate ID column, which should make it relatively simple to sum up each affiliate's total earnings during the selected date range.
If you need to compose a
masspay.txt file to pay out older commissions, you can follow the format of your latest masspay file and/or refer to this help page explaining the PayPal Mass Payment file format.