Hi @Murat Kilci -- Cam from the Intercom Support Engineering team here 

This type of error often occurs when there’s an issue with identifying the end-user thats sending the Message.

Presuming that you’re interacting with Messenger as a logged-in User - if you have the “Enforce Identity Verification” capability switched “On” for your website and/or mobile app (which we STRONGLY recommend you do - setup details here), but the Intercom installation being used in the platform where you’re interacting with Messenger has not been setup to handle the Identity Verification process for logged-in Users then it can result in this type of error. 

I’d suggest checking these details within your Workspace Settings, and in the integration/installation being used in your site to confirm if things are setup as needed