Each plan has a maximum number of boards you can create.  Please refer to https://www.sortd.com/pricing for more information.

If you are unable to create a new board and you see a message saying 0 boards available, you will either need to delete one of your existing boards first or upgrade to a different plan.  To upgrade to a different plan you can just click on the get more boards link above the Organization dropdown.

You can also delete a board if necessary by going through the board settings.