This special configuration only applies if your firm has chosen to utilize Multiple Business Entities.
If this applies to your firm and your accounts with the firm, you will see the following information in the Client Portal when viewing your outstanding invoices:
You can only pay invoice(s) from one business entity at a time, as highlighted by the note in yellow. This is because those funds must go into different bank accounts and separate payments is the only way to ensure this happens.
Simply select only the invoice(s) from one business entity to load them up into the Summary checkout section and proceed with making the payment.
After you make a payment, you're brought back to the listing of your outstanding invoices, with the remaining invoice(s) shown and able to have a payment made. Select them and again proceed with making your payment.