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BigPicture requires administrative rights to function properly. To start using the BigPicture Trello Power-Up, please ensure the Team’s Administrator has already launched it.
Solution
An admin of the organization must start the App (and use it - click something while the App is running).
If the administrator who initially started the App lost their Admin rights, another admin must start the App again.
This must be done by the admin of the Trello team (not the admin of the board!).