Mac accepts only 24 recovery key characters

A FileVault Recovery Key is 24 characters, while an Apple Account Recovery Key is 28 characters.

Given where you're posting this is not the Apple Account community, I'll assume the forgotten password is the macOS login password, and not a forgotten Apple Account password.

Here are the options for resetting the macOS login password:

• If you forgot your Mac login password - Apple Support
• If you can't reset your Mac login password - Apple Support

It's possible your Apple Account and login can be used to reset your macOS login. But depending on what you've forgotten and what you still recall, you may need to erase the Mac storage and re-install macOS.

If you forgot your Apple Account password, and have set up a Recovery Key, then that does need the 28 character Apple Account Recovery Key. The Recovery Key blocks other means of performing an Apple Account password reset, too. That's the whole purpose and fundamental reason for a Recovery Key: it blocks other means of performing a reset.

The first of the following Apple Account password reset links is the general sequence, while the second describes using the Apple Account Recovery Key, and the third is included but is probably not available as that'll get blocked by the use of a Recovery Key.

• If you forgot your Apple Account password - Apple Support
• Set up a recovery key for your Apple Account - Apple Support
• How to use account recovery when you can’t reset your Apple Account password - Apple Support