Thanks for getting back here, @AdminE. I want to ensure you'll be able to turn off the automatic invoice reminder in QuickBooks Online (QBO). 

In QBO, a user with the company admin role has access to every part of the account. They can do everything a primary admin can do, apart from editing or removing its access. You'll know about this by visiting this article: User roles and access rights in QuickBooks Online. 

Since you're having trouble locating the Automatic invoice reminders option, let's perform some troubleshooting steps to determine if this is a browser-related issue. The initial step is to access your QBO account via incognito mode. This does not store cache files that may cause odd behaviors such as missing options when running web-based programs. Below are the shortcut keys you can use:

• Google Chrome: press Ctrl + Shift + N
• Mozilla Firefox: press Ctrl + Shift + P
• Microsoft Edge: press Ctrl + Shift + P
• Safari: press Command + Shift +N

Once signed in, go to the Reminders section again following the steps of my colleague above. From there, check if you can now see the Automatic invoice reminders option. Please refer to the screenshot below: 

If this works, go back to your regular browser and clear the cache to eliminate piled-up data. If the same thing happens, you can use another supported browser or device. Might be that the current one you're using has a temporary issue with QBO. 

Once everything is fine, your next step is processing customer payments for their invoice in QBO. You can use this link as a reference: Record invoice payments in QuickBooks Online. 

I'm always here ready to lend a hand if you have any other concerns about managing invoices in QBO. Please don't hesitate to leave a reply below. Have a good one.