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Dear Jonathan Ridgway,
To better understand the issue, could you please provide a little more detail about the exact steps you are taking when you try to add or convert the member to an owner? In particular, please let me know where you are managing the group, for example, from the Microsoft 365 admin center, SharePoint admin center? Please take a screenshot showing the error then share it with me.
As an additional test, could you please try changing the member to an owner directly from the Microsoft 365 admin center, as shown in my screenshot?
- Open the Microsoft 365 admin center.
- Select Teams & groups > Active teams & groups.
- Select the affected Microsoft 365 group.
- Open the Membership tab.
- Select Members.
- Select the member you would like to promote.
- Select Make owner.
Please correct me if I'm misunderstanding. Thank you very much for your valuable time and your cooperation.
Looking forward to your response!