I'm also having trouble with this step.
I'm curious if the "If you need to edit off any subdomains to make it work, please do so here" text field actually does anything. I originally started the test with a domain which is in active use so I can't really mess with it too much (such as moving the whole thing to an IPv6 capable DNS provider!), so I added a subdomain that's a substring of the original domain. I then added an NS record for that subdomain that delegates it to HE DNS (ns2/ns3) and registered the subdomain in HE DNS with a valid AAAA record.
I'm trying to use this new subdomain in the test by entering it into that text field, it passes Step 2, but Step 3 always says "Couldn't query name server ". Everything seems fine when I try from the command line, so I'm wondering if the domain edit field is actually editing the domain that's tested? Has anyone successfully used it before?
I seem to be stuck at the same point as well.
When I dig: ipv6.stevenmiano.com I get a AAAA record back, and using dig @2607:f208:206::12 aaaa ipv6.stevenmiano.com it seems to respond correctly as well.
Would anyone have a hint or tip as to what I might be doing incorrectly (or not at all)?