When Trying to Change Permission for App, an Error Is Displayed Indicating That There Is No User Who Can Manage the App

If you click Save in the Permissions for App when there is no app administrator, an error is displayed indicating that there is no user who can manage the app.
To fix this error, you need to specify a user/organization/group who manages the app.
Detailed steps are as follows.

If No Checkbox Is Selected for the "Manage app"

Select a "Manage app" checkbox for a user/organization/group who manages the app.

If It Is Considered That No App Manager Exists

Even if a user's checkbox for the "Manage app" is selected, that user might no be considered as the app manager depending on the access setting.

If you select multiple access permissions for a user, the setting of the upper line has higher priority.
Suppose that you do not grant app management permission to the sales department, while granting that permission to Barbara Miller who belongs to the sales department.
In this case, Barbara Miller is not considered to have app management permission as well. Therefore, she cannot save the permission setting.
In order to save the permission setting, drag the arrow button that is in the far left of Barbara Miller's row and move that button to the top.