Thank you for the screenshot you've added. I'm here to ease your confusion today by sharing the steps on how to add your sub-account successfully.
From the Is sub-account drop-down, you can select an expense account or the parent account where you want to add this sub-account you'll be setting up. If you're still having difficulties saving the account, I recommend trying the steps using a private window to check if it's a browser issue or not.
To save you time, use either of these keyboard shortcuts:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
If it works, you'll need to go back to your original browser and clear its cache to start fresh. If you get the same result while using an incognito/private browser, I recommend switching to a different browser.