Let's perform some troubleshooting steps so you can save your EIN. There are times that the browser stores frequently accessed data that can cause QuickBooks Online not to function well. Let's start with logging in to the system through a private browser and enter and save the EIN from there.
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Option + P
If this works, it means that you need to clear your browser's cache so the system can start fresh. If you get the same result while using a private browser, I recommend switching to a different browser.