Hi David,
Thanks for your post here.
I understand that you encountered the issue when you want to add your domain in Office 365.
Based on the error message you provided, it seems that you’ve already have a website which with the domain for SharePoint website in Office 365. from the screenshots you’ve provided, I assumed that you’re using P Plan(Office 365 for Professional and Small Business subscription) please correct me if I’ve misunderstood. To solve the issue, I suggest you to try the steps below:
- Login in your domain control panel in your domain provider, change the Name Server to ns1.bdm.microsoftonline.com and ns2.bdm.microsoftonline.com. for details, please refer to: Verify your domain and change name servers.
- After done, in the “Domain” panel in Office 365, check the SharePoint website address above the domain list, if it’s using your domain name, click “Change address” to change it back to default contoso.sharepoint.com.
- Wait for the change complete(you can check that in the domain panel in Office 365). When done, then you can add your own A or CNAME record in “Manage DNS”, for details, please refer to: Update DNS records to keep your website with your current hosting provider．
Please feel free to post your updates here at your convenient time.