想通了.显然，当我尝试发送到的 FCM 令牌不再注册时，就会发生此错误，如 "messaging/registration-token-not-registered" 错误代码所示.在这种情况下，我只需要从用户的令牌中删除这个令牌并完成它.

Figured it out. So apparently, this error happens when the FCM token I'm trying to send to is not registered anymore, as evidenced by the "messaging/registration-token-not-registered" error code. In that case I just need to remove this token from the user's token and be done with it.