I am attempting to pay an online payment to a vendor and the dropdown menu for which account to pay from will not do anything. Ultimately, I am blocked from paying.

Try opening your QBO account on any private/incognito browser.

Welcome to the Community space, @Troy37. I'm connecting to provide troubleshooting to help you process an online payment to a vendor inside QuickBooks Online (QBO).

When accessing the program via a web browser, you may notice changes in your company file's functionality due to temporary internet data. The usual culprit is the temporary internet data that the browser accumulated from past or recent browsing activities. You'll want to access QuickBooks using an incognito or private browser to prevent browser-related issues. Here are the shortcut keys to open one:

• Safari 11 or newer: ⌘+Shift+N
• Chrome: Ctrl+Shift+N
• Safari: Command+Shift+N

Once you're in, enter a bill for the vendor and pay it. Choose a bank/Credit card account to settle the bill. If you're able to proceed, clear your browser's cache. Refer to this article for the steps: Clear cache and cookies to fix issues when using QuickBooks Online.

You can also use other compatible browsers if the issue persists. Refer to this article to see the list of compatible browsers in QuickBooks Online: Other browsers.

Also, I recommend contacting our Customer Care Team regarding being blocked from paying a vendor using the online option. This way, they can check your account and help you get through this as soon as possible. Refer to this page for more information: Get help with QuickBooks products and services. 

On top of that, I'm adding this article to help you learn more about how payment processing works inside the program: Learn about QuickBooks Bill Pay for QuickBooks Online.

I'll always be around to extend assistance in handling vendor payments inside the program or if you have other questions about QuickBooks. I'll make sure to respond as soon as possible. Keep safe.