You're right, you must be an admin of the board to close it.
When you open the board's menu and click the "More" button to expand the menu options, do you see an option to Close Board ?
If not, could you share a screenshot of the board menu where the Close Board option is usually available? Happy to take a closer look!
If you had to thrive a new habit during a lockdown, what would it be? Trello
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