This is probably very clearly stated in the directions but it made me a little nuts because I was working in Safari and Chrome at the same time. One was logged in to my developer account and the other to … nothing.
So it turns out that when you are testing you must be signed into your development account (for example, in another tab) before you can use your test buyer. Shouldn’t I have known this? I probably did already know it, and just forgot, it’s not often that I go back to revisit the Sandbox.
Hope it helps if you’re having trouble!