Hi there, h777.
I appreciate the steps you've performed with our tech support. With regard to the compatibility of QuickBooks Desktop 2020 and merchant processing using Windows 8.1, you can check System requirements for QuickBooks Desktop 2020 and Enterprise Solutions 20.0 that contains information about the operating system, hardware, software, and other requirements to run QBDT.
I've also added some links to learn more about QuickBooks Desktop 2020:
Feel free to post again here if you have any other concerns.
