I assume you're trying to reset the password using your Apple ID? Follow the steps at section "Reset using your Apple ID" described on support page: Change or reset the password of a macOS user account - Apple Support
If that doesn't help, try section "Reset using the Reset Password assistant (FileVault must be on)" or "Reset using your Recovery Key (FileVault must be on)" on the same support page.
(Be sure to check you 'Caps Lock' setting...)