Hi @정해윤 ,
The unlicensed user in Office 365 can't use most of the service: OneDrive, Office online, Exchange online etc. The user needs the corresponding license to use the corresponding service.
Since your issue is related to Yammer, which is not supported on Q&A forum currently, I would suggest you post a new thread on Microsoft Tech Community.
Tech Community --- Yammer Thanks for your understanding.
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