Please set according to the following page
https://icts.nagoya-u.ac.jp/en/services/nuid/CAS/mfa_cas_first_time_seed_registration.html

If you remember setting this up in the past, you probably did not complete the setup correctly, so please try again.

Or, you may be confusing multi-factor authentication settings from another authentication service with CAS multi-factor authentication.
There are several systems that require multi-factor authentication settings, as described in the following pages.
https://icts.nagoya-u.ac.jp/en/services/nuid/CAS/mfa-systems.html