Forgot Macintosh HD disk password

See:

Use FileVault to encrypt the startup disk on your Mac - Apple Support

"Reset your password or change your FileVault recovery key

If you forget your account password or it doesn't work, you might be able to reset your password.

If you want to change the recovery key used to encrypt your startup disk, turn off FileVault in Security & Privacy preferences. You can then turn it on again to generate a new key and disable all older keys."

If you still can't recover it you might have to erase your hard drive, And do a clean install of Mac OS losing all the data you have not backed up

Good luck!

Then you might have to erase the startup disk.

For further help you might want to:

Contact Apple for support and service - Apple Support

Good luck!