Hi @IBN
From a similar post https://techcommunity.microsoft.com/t5/microsoft-teams/unable-to-unblock-a-user-quot-we-can-t-unblock-this-person-at/m-p/3607006:
Messages go through for other users, just not the user that was blocked. I ended up opening a ticket with MSFT and turns out it was a bug in our DoD tenant.
I recommend you go to https://admin.microsoft.com/ and open a ticket with Microsoft Support as there might be an issue with your tenant.
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