Your Pi-hole has two-factor authentication enabled. You have to enter a valid TOTP token in addition to your password. You see this message because your token was incorrect or has already expired.
After installing Pi-hole for the first time, a password is generated and displayed to the user. The password cannot be retrieved later on, but it is possible to set a new password (or explicitly disable the password by setting an empty password) using the command
pihole setpassword