I had the same problem when creating my administrator accounts 3 weeks ago. I chose a password under 15 characters, but there was no error message, and I had to puzzle over it. After I chose a longer password, the account was created.
Sorry if it’s not clear - this is the initial signup given to an admin signup, I believe this is only available through a self-hosted site, not one through Discourse’s hosting.
There’s a completely separate sign up form after you get the “congratulations you installed Discourse”, this is the full screenshot of the signup page in question: