I haven't done this with a Creator license so it might be a bit different but here's a couple of things I'd try:
1. If you are trying to join an existing Tableau Online site, request the Site Administrator to send you an invite.
2. If this is a new Tableau Online site (i.e. if your creator license is the first one for your organization), go in your browser to online.tableau.com and click 'sign up' -> 'Free Trial' and login with the e-mail registered to your license. This should create a site and you should get an e-mail to activate the site. Then you can login freely.
3. If you run into any issues during this, create a support ticket here: Support Case | Tableau Software. You may also start here if you'd prefer.
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