If the tax verification form in Step 4 is not loading for you, please try the following:
Use a different browser - It's possible something in your browser is preventing the embedded form from loading, such as an ad-blocker or other extensions/add-ons. Make sure your browser is also updated to the latest version. You may want to also try clearing the browser's cache and cookies as well.
Use a different device - If you are using a work issued device, it's possible your employer might be blocking access. Switching to a personal device should help as well as making sure you are not connected to a VPN.
Note: this is a different, known limitation — if you added another parent/guardian to your application after previously signing or submitting your application, you will not be able to edit the form.