By default, when you load a page that requires authentication (a client account, for example), or a conversion tunnel page, the page displayed in the editor will be empty or show an error, because it often requires session information (ex: products to be displayed in the cart page).
You can use the "Apply source code" option to get around this issue. This feature is present in the Main information of the test:
To load your page in the editor, paste the page’s source code in the window. When you navigate to the page you wish to test, push “Ctrl + U” to display the source code. Copy the page’s entire source code into the Apply source code field.
Click on "Save" to validate it. The page will now load in the editor, and you can apply your modifications to this special type of page.