Users who are group administrators or sub-administrators cannot be deleted.
Remove their administrative privileges before deleting the users.
When Deleting a Group Administrator
A group cannot be left without an administrator.
First change the group administrator to a successor user.
Related: Change group administrator (Web Browser)
https://teachme.jp/8/manuals/9030824
When the group administrator is changed, the previous administrator automatically becomes a sub-administrator.
Remove the sub-administrator privileges before deleting the user.
Related: Remove group sub-administrator (Web Browser)
https://teachme.jp/8/manuals/9030830
When Deleting a Group Sub-Administrator
Remove the sub-administrator privileges before deleting the user.
Related: Remove group sub-administrator (Web Browser)
https://teachme.jp/8/manuals/9030830
Related
Delete user from group (Web Browser)
https://teachme.jp/8/manuals/8930266
Delete multiple users at once using CSV file (Web Browser)
https://teachme.jp/8/manuals/4965298