Hi Kojiro,
That is odd as the authentication is handled by your iPhone’s Home.
I’d suggest this to hopefully resolve the issue:
-is your iPhone updated with the latest 15.4.1?
-is your HomePod updated to the latest 15.4.1?
-if both are updated, try restarting your iPhone.
-If you go into your iPhone Settings>your name, at the bottom it shows the ‘sign out’ option (which means you’re signed in)? Signing out then signing back in may be worth a try (be certain you have the correct password, see the **two factor authentication bullet point below).
-On that same page, if you press ICloud, you have Home enabled for iCloud?
-In the same iCloud menu, do you have keychain enabled (I’m not certain you need this, I’m relaying my own settings for my HomePods).
**If you login to your Apple account and go to security, you have two factor authentication on? https://appleid.apple.com/account/manage/section/security
If none of this helps, I’d suggest a reset->a fresh install of the HomePod. Reset HomePod - Apple Support