Hi Anders,
Before adding a domain to a new tenant of Office 365, we should ensure that the domain had not been added to another tenant or has been removed from the tenant.
Although you have deleted the accounts of you and your partner, another admin account that applied for Office 365 service before can’t be deleted and will be always kept till the tenant expires.
Please use the initial admin account to log into Office 365 and remove the domain artcgroupionc.com at first.
Please try your best to recall the username of the account, it may be xxx@artcgroupionc.com or xxx@artcgroupionc.onmicrosoft.com, the password can be reset via the alternate email address.
Thanks,
Jacky Ding