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.
Reference link:https://learn.microsoft.com/en-us/graph/api/chatmessage-post?view=graph-rest-1.0&tabs=http
Please check and validate the same from your end.
Hope this helps.
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