When you purchase the enterprise edition annual subscription, you get 10 free active anonymous forms with the package. Once you have published them all and try to publish a new one, you’ll get such a message.
If you don’t have all the 10 forms published or used, you can unpublish and delete them using SPARK Forms Manager, to allow space for the new form.
If you need extra anonymous forms, you can pay for another 10 anonymous forms.
We are talking here about anonymous forms only, as your internal use forms are unlimited.