When attempting to add a user from the directory the subscription is using I get the message "The co-administrator must be a Microsoft account, or a user in the XXXXXXXX directory. You can only add a co-administrator from the XXXXXXXX Directory if you are also a user in that directory.
- I logged in as the account administrator (not a co-administrator)
- I clicked New - Add Directory - Use Existing Directory - Signed Out
- Signed in using the onmicrosoft.com credentials of the Directory I am trying to add
- Added the Azure "account administrator" account as a global admin to the active directory that is synced with our on-prem environment.
- Logged back into Azure portal as the account administrator
- Went to Settings - Subscriptions - Edit Directory - Changed the directory to the AD that is synced with On-Prem
- Went to Administrator and tried to add an account (that I verified through the AD part of the Azure portal) and got the error above.