So, for the new owners. Don't try to activate the app until you get an e-mail from Honda. It seems like it's a manual process on the back end.
Or call HondaLink support. I have never had to call them as mine has worked pretty much flawlessly from day one since I got my Clarity last April. But people who have called HondaLink support say they can normally fix whatever it is, although like with any phone support the experience can be frustrating depending on who you talk to and what the particular root cause problem is. But if it's just resetting an account they can usually handle that fairly quickly in most cases. Although some people are unlucky and problems take longer to fix and require multiple calls.
To be more specific when I say my HondaLink has worked flawlessly, I mean for what I mainly use it for which is charge scheduling, which is just about every day, and remote climate which I use maybe a few times per week. I have a time of use rate from 11:00pm to 7:00am so I schedule my charging during that time based on my estimated driving for the next day, as I don't like to charge to full if I don't need to. Now that's a big topic of debate and whenever I make that statement invariably someone will pop in and lecture that 100% is not full as there is a buffer above that, which I know, but the topic is not that simple and anyway is OT from the current discussion. I will just say that personally I don't charge to full unless I need to, and I bring it up here only to make the point that unlike probably most people I use scheduled charging almost every single day and it is extremely rare when it won't connect, in those rare cases I can usually try again a half hour or so later and it's fine.
Now when I first got the car there were some parts of HondaLink that did not work for me, but they were features that I didn't really plan to use but was just curious and wanted to try them. Send destination to map is one, but after hearing how it works it's a joke, a very poorly implemented feature even when it is working, and I use Google Maps anyway so no real loss.
And although I can almost always initiate Turn On Climate with no problem, I could never get scheduled climate to work, it would not allow me to enable it (I forget what the error message was). I didn't call HondaLink support because it was a feature I don't really need since my car is in an attached garage and I live in Atlanta which does not have extreme weather. But out if curiosity I just wanted to try it, so every time they had a new update for HondaLink I would try scheduled climate, but still could not enable it. Finally I don't know maybe last September or something there was an update, I tried scheduled climate and for the first time it worked. Curiosity satisfied, I haven't used it since. I am just bringing it up to point out that the different features work differently and it's not uncommon for some people to have a problem with certain features whereas other features work fine for them. If nothing works however then it's usually an account problem.