Understanding pam module

Hello everyone.

I'm getting an error when attempting PAM authentication.
I'd like to understand what PAM expects, how it expects the password, 
and how it expects the OTP.
The debugging step is below:

(0)   Auth-Type PAP {
(0) pap: Login attempt with password
(0) pap: Comparing with "known-good" SHA-Password
(0) pap: User authenticated successfully
(0)     [pap] = ok
(0)     update request {
(0)       EXPAND %{Tmp-String-0}%{Tmp-String-1}
(0)          --> password123456
(0)       User-Password := password123456
(0)     } # update request = noop
(0) pam: Using pamauth string "radiusd" for pam.conf lookup
(0) pam: ERROR: pam_authenticate failed: Authentication failure
(0)     [pam] = reject


I use Google Authenticator.
Thanks. Marco.

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