You'll only be able to move a team board to public if you are the Owner of the Team.
Note: Moving team boards to public will make them accessible to anyone with the URL. Also, it will count as a new public board. Meaning: This will count on your limit of public boards for the month.
Here are the steps you need to follow to move a team board to a public board:
Step 1. On your Dashboard page choose the team board you want to move by clicking on the 3 dots icon (ellipsis)
Step 2. Choose the option “Move to public”
Final Step. You’re going to see a message that says: “Moving this board to public will make it accessible by anyone with the URL, also it will count as a created public board and will count on your limit, are you sure about it?”
Now click on the button “Yes, move it!” to move your team board to public.