Hi @Vicky Horbury ,
You cannot join that board because it was already public. But if you can contact to the admin/creator of the board and request them you want to access the board as a member you can do that!
In the current situation, you can also copy the board to your own Trello account to use it as you want. Here are the steps to copy the board:
Click on Show Menu -> More... -> Copy Board.
Here are some extra information about Copying cards, lists, or boards
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