Based on my knowledge, Global admins can convert an existing team to an org-wide team by editing it in Teams client. Go to the team name, click More options > Edit team. If you have this error message, we recommend you try to check these settings:
1)There’s also a limit of five org-wide teams per tenant. If these requirements are met, global admins will see Org-wide as an option. So please check it.
2)Check if you have the same problem in Teams web client. If there is no issue in Teams web client, please try to clear the client cache file.
3)Try to change another Team to see if this issue can be reproduced.
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