I was in the same situation like you after followed the same guide.
My mistake was I didn’t filled out the IFTTT API URL in the General section of the API tab. We must set our Custom Openhab URL and this is not covered in the guide.
Now, the Auth test pass all green, I hope this is the same mistake for you.
I’ve faced to the same situation, stuck in loading state in item selection.
I’ve managed to get it work again by unlink my account in the setting of My applet > Services > My custom service and relink it again.