Does this user have the auto-expanding archive enabled?
Once the auto-expanding archive has created an auxiliary archive space, you can't delete folders in the online archive: https://docs.microsoft.com/en-us/microsoft-365/compliance/autoexpanding-archiving?view=o365-worldwide
Also, check this link for help - https://learn.microsoft.com/en-us/microsoft-365/troubleshoot/archive-mailboxes/cannot-delete-folders-archive-mailbox