Lock not available for notes

Which system version do you have installed? You can lock Notes on macOS 10.13 High Sierra or later.

Did you assign tags to your Notes? Then you can no longer lock them: Lock your notes on Mac - Apple Support (IN)

"You can’t lock any of the following:

• A Quick Note
• A note you’ve shared with other people
• A note that has a video, audio file, PDF or document attached. Only tables, images, drawings, scanned documents, maps or web attachments can be included in a locked note.
• A note that contains tags
• A note stored in an iCloud account that hasn’t been upgraded. See About using iCloud Notes."

See the article How to lock notes on your iPhone, iPad, and iPod touch - Apple Support

If you're signed in to iCloud with a Managed Apple ID, you can't lock your notes.

What is managed Apple ID account see the article About Managed Apple IDs for business - Apple Support

Check feature availability

Your employees can use Apple services including iCloud and collaboration with iWork and Notes, but because Managed Apple IDs are for business purposes only, certain features are disabled to protect each organization. Managed Apple IDs also include 5GB of iCloud storage.

Because Managed Apple IDs can't make purchases, administrators may assign content to your Managed Apple ID or assigned devices.

For Mac Lock your notes on Mac - Apple Support