Lock notes are blank

May be notes application and first admin account is not working , for the time being create a new admin account in users and groups Change Users & Groups settings on Mac - Apple Support and set up iCloud notes Set up iCloud for Notes on all your devices - Apple Support

Observe new account see if notes application is working .

See the article Lock your notes on Mac - Apple Support

Lock a note

After you set up a password, you can lock your notes.

1. In the Notes app  on your Mac, select the note you want to lock.
2. Click the Lock button , choose Lock Note, then enter the password. (You can also use Touch ID on your Mac or Magic Keyboard if you turned on that option in Notes settings.) If you haven’t created a password yet, you’re prompted to do so.
3. If you’ve already entered the password during this session of Notes, you don’t need to enter the password.
4. After you lock a note for the first time, all locked notes in that account are unlocked so you can easily view them.

If problem persists contact apple support Contact Apple for support and service - Apple Support