Hello @Isabelle Tot ,
Currently the Core Boards used by the Business Project type do not display the Subtasks in the board.
To get this functionality you would want to look into a Jira Software project type and the Swimlane capabilities of the board configurations on Software Projects.
Also, we have the following Feature Requests tracking interest in this as a new capability for Core Boards, make sure to add a vote to help us track continued interest and influence the priority as covered int he "Implementation of New Features Policy" :
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