PlugSignature.failure

You're seeing just the function failure, go back to PlugSignature module for more information.
Link to this function

failure(conn, reason, algorithm, headers)

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Specs

failure(Plug.Conn.t(), String.t(), String.t(), String.t()) :: Plug.Conn.t()

The default failure function.

It logs the failure reason, returns a 401 'Unauthorized' response with a 'WWW-Authenticate' response header listing the supported algorithms, and halts the connection.