Each plan has a maximum number of boards you can create.
The Pro plan allows you to create a maximum of 3 boards (that only you will be able to see and use), the Team plan gives you 5 boards that you can share, and the Team Plus plan has unlimited shared boards.
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.
To delete a board, go to Settings > Account > Click on the organization you want to create the new board on. Then click on Boards and you will see a Delete button.