can't open locked notes on imac

You've upgraded your notes, correct? And you're certain that you're running the latest version of MacOS? I've never locked notes, but I tried one now and I cannot recreate your issue.

Upgraded iCloud notes

If there’s an Upgrade button next to iCloud in the sidebar of the Notes app, you haven’t upgraded your notes yet. This means that until you upgrade, you can’t apply paragraph styles or checklists, browse the attachments in your notes, see and restore recently deleted notes, lock your notes, or add people to notes.

If you upgrade your notes, you can view and edit them on other devices with OS X 10.11 or later, iOS 9 or later, or iPadOS 13 or later that are signed in to iCloud using the same Apple ID. You can’t access upgraded notes on devices with earlier versions of OS X or iOS. Also, any notes you create or edit on those devices won’t appear on devices with upgraded notes.

When you upgrade, most of the formatting in your notes stays the same. However, any notes with HTML formatting (notes you created by copying and pasting from a website) may look significantly different.

FWIW — I’d probably provide feedback at Apple.com/feedback and I’d stop locking notes if your concerned about access.

The data in locked note created from iPhone won't sync with Mac when signed in with same Apple ID and password on the same network , however notes created in normal way will sync both ways ( created from iPhone or Mac ) , send your feedback Feedback - iCloud - Apple

You can unblock the note from iPhone .

A password is required to create a locked note , if you have incorrect password the note can never be unlocked , the normal notes is create a note from note application , that is not having any password for creating it , it will sync between iPhone / Mac .

See the article Lock your notes on Mac - Apple Support

See Lock your notes on Mac - Apple Support