Let me route you to the best help available, @reginang00.
Once you get denied from your payment processing application, I'd encourage you to contact our Payment Support Team. This way, they can further check on your application and provide another way to process payments in QuickBooks Online.
You can also refer to our Merchant Agreement and Acceptable Use Policy articles to know some reasons why your application is denied.
You might want to learn more about processing payments in QuickBooks Online. This article will provide detailed information: Take and process payments in QuickBooks Online.
Please keep in touch if there's anything else I can do to help you succeed with QuickBooks. I've got your back, @reginang00.