May I know what are you trying to do when you get this message? Also, if you haven't tried, let's use a private browser when accessing QuickBooks Online. This way, we would be able to isolate the issue if this is a related browser or not.
When you get to access your account using a private browser, let's clear the browser's cache. It helps clearing historical old files causing the program's performance to be jammed.
Let's access your account and create a sales receipt using a private browser or an incognito window. This way we can isolate this error for a possible browser-related.
Here are the shortcut keys you can use to open a private or incognito window from any of our supported browsers:
Chrome simply press Ctrl + Shift + N on your keyboard.
Mozilla Firefox and Internet Explorer, press Ctrl + Shift + P.
Internet Explorer: press CTRL + Shift + P
Safari, press Command + Shift + N. Make sure you're using the latest version.
Once you're able to create a sales receipt successfully, let's open the regular browser you're using so we can remove its cache and temporary file. The stored data in your regular browser may be corrupted and results to unexpected behaviours. If you're still unable to access the transactions input key, try using a different browser.
On the other hand, I've added these articles to help you get your way around QuickBooks: