BigPicture requires administrative rights to function properly. In order to start using the BigPicture Trello Power-Up, please ensure that it has already been launched by the Team’s Administrator.
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, then another admin has to start the App again.
Must be done by the admin of the Trello team (not the admin of the board!)