The One-Time Password (OTP) unlock feature is obsolete in SecurityCheck Pro: it was removed in version 5.2.0 and no longer works on current Pro releases.
In SecurityCheck Pro, OTP has been replaced by the email rescue link, a safer recovery mechanism that does not require multi-factor authentication to be configured on the account. If you have been blocked by the firewall, see Rescue link - I have been blocked; how do I sort this? for the current recovery procedure.
If you are running a Pro version older than 5.2.0, please update: older releases no longer receive fixes.
The free Securitycheck component still includes the OTP unlock: visit https://www.example.com/administrator/?username=admin&otp=123456 (replace the username and the code shown by your MFA app). It requires the Verification code or Yubikey MFA method to be active on the locked-out account.