OK, I don’t know what happened when I first installed Anki on my iMac a couple of days ago, but I was not asked to set up an account or informed of any login details. So today when I tried to start Sync to set up Anki on my iPad, it said I had to log in—and none of my normal logins worked.
I’ve checked the various help links, and not found anything relevant. In order to post this question, I set up a new account, which is obviously not linked to my Anki data.
You can use Anki locally on your device without any account. Before you can sync, you need to create an AnkiWeb account – Syncing with AnkiWeb - Anki Manual .
Thanks, Danika. I see now that my problem was that when I clicked on Sync and saw the login screen, I thought that meant I was supposed to have an account already. I didn’t notice the “Sign up” link. I’ve now done so, and was able to continue with the sync process. Thanks for pointing me to the intro video on syncing, which I found very useful.