Event 6273 with reason code 8 — "The specified user account does not exist." NPS parsed an account name out of the RADIUS request and could not find a matching account in the domain it searched. This is an identity-lookup failure, before any password or certificate is even checked.
Reason code 8
NPS 6273 reason 8: the specified user account does not exist
What it means
Common causes
Username format mismatch: the client sends
host/machine.domain.com,DOMAIN\useroruser@domainin a form NPS cannot map to the account.The request carries a realm prefix/suffix (common via wireless controllers) that needs stripping before lookup.
Machine authentication from a computer that is not domain-joined (no computer account to find).
The account genuinely does not exist — deleted, or the wrong domain is being searched in multi-domain forests.
How to fix it
Look at the Account Name field in the 6273 event — that exact string is what NPS looked up. Work out why it does not match an existing account.
Use attribute manipulation in the connection request policy to strip realm prefixes/suffixes if the NAS adds them.
For EAP-TLS with certificate names, ensure the certificate's identity (UPN/DNS SAN) maps to a real account in the expected domain.
In multi-domain forests, register NPS in each domain and confirm the NPS server can resolve accounts across trusts.