@서영민 파트장 (Youngmin Seo, 徐榮敏) , Thanks for posting this question and sharing the screenshots.
Just to confirm, are you referring to this tutorial for prerequisites
and steps outlined on setting up chat?
If you haven't checked this already, kindly ensure that the participants on the chatthread need to be valid users.
Also, please confirm that you have replaced proper ids from the samples. E.g. And have it as- CommunicationUser user1= communicationIdentityClient.createUser()
There are similar discussions on this, please see these:thread1 and thread2.
I'm also discussing on this internally. Please let us know how it goes, I'll follow-up on this soon.