Hi @BERKAY ŞAHİN
Hope you are doing well.
Attachment's ID in Adaptive card can be a new randomly generated GUID and must be unique. However, the attachment's ID must be the same in the body and attachments elements. ID is required when sending a card as a message.
I have executed the same example provided at: https://learn.microsoft.com/en-us/graph/api/chatmessage-post?view=graph-rest-1.0&tabs=http#example-3-send-message-containing-cards with my own randomly generated ID in GUID format. Card message has been successfully posted on Team's Channel. Please find the below screen shot.
Please check and validate the same from your end.
Hope this helps.
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