Welcome to the forum, @Raaaan!
Can you please clarify what you mean when saying that your plan is just inactivated?
Were you able to successfully cancel your App Store subscription with these steps? What plan do you see on this page?
In any case, please keep in mind that you'll need to wait till your Dropbox account has been downgraded to a free Basic one and it's no longer on an in-app subscription, before upgrading to Dropbox One.
Let me know if you need further help, though.