QB Issue Resolution:

Are the invoices due? (check the due dates) Are the invoices marked as pending? Unmark them (its in the edit menu when the Invoice window is up). Do you have multiple Accounts Payable type accounts? If so, try selecting other ones (the pop up will be in the pay bills window if you have multiple types).

Resolution for Issue 'Open invoices are not showing up in the payment window' available: Yes (Solved).
Source: Intuit Community forum.

Open invoices are not showing up in the payment window: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Invoice
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Error Recovery for Invoice [Canada]
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