By default, Flexi only offers advance payments that have already been paid for deduction. If you need to deduct an advance payment that hasn't been paid yet, you must first enable this in the company settings.
Procedure
Go to Company → Settings → Modules tab.
For issued advance payments: Sales tab → Invoice tab → check Allow deduction of unpaid issued advance payment.
For received advance payments: Purchase tab → Purchase tab → check Allow deduction of unpaid received advance payment.
Save using the Save and close button.
ℹ️ This setting applies to the given company and is valid for both cloud and on-premise servers. It is enabled separately for sales and for purchases — if you need both, check both options.