Hi @Mike Landi
According to the Graph Api, Application permissions are only supported for migration. This error is therefore reported if you send a message while chatting with application permissions.
When sending a message in a chat, you need to use the delegation permissions ChannelMessage.Send or Group.ReadWrite.All, which will send the message in the chat on behalf of the logged in user, and you need to get a token through auth code flow when using the delegation permissions. You can refer to this document.
Hope this helps.
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