Edit: Oh no, indeed when you want to sign up you need an sso provider indeed! This is what they say:

Can I sign up with an email address?

We don’t support sign-up with email addresses. By design, Tailscale is not an identity provider: there are no Tailscale passwords.

Using an identity provider is not only more secure than email and password, but it allow us to automatically rotate connection encryption keys, follow security policies set by your team (e.g., 2FA), and more.