Default structure is selected when the user opens Structure Board for the first time, or when the Auto-switch option is set to default structure. You can change the default structure for the JIRA instance and for a specific project.
The new system-level default structure will be also default for all structure-enabled projects that don't have this setting overridden.
Make sure that default structure has correct permissions. If the structure is selected for the user by default, but the user does not have VIEW
access to it, the user will see an error.
Project administrator can also change project-level structure from the Structure tab on the project administration page, or from the options pop-up window on the Structure tab on the user's project page.