Why do I get "Preview Hidden" on message app on my Mac laptop?

Go step by step and test.

1. Restart the Mac.

2. System Preferences > Notifications > Messages > Show Preview

Choose "Always".

https://support.apple.com/lv-lv/guide/mac-help/mh40583/mac

Quote:

Show previews

Show a preview in notifications for the app. For example, if you receive an email, you see a preview of the email in the Mail notification.

• always: Previews are always shown (even in the login window).
• when unlocked: Previews are shown only when you’re logged in to your user account.
• never: Previews are never shown.

Note: If you choose a setting from the “Show previews” pop-up menu at the bottom of the preference pane, it’s applied to all apps.

SkyEllenFielding wrote:

This ONLY applies to the message app on my Mac, NOT on my iphone. It only happens with one person, who is on a Samsung (new). It doesn't happen with other people who are not using iphones. I just want to be able to see messages on my laptop as well as my phone. Any ideas?

ref: https://www.macworld.co.uk/how-to/get-text-messages-3582821/

Use Messages with your Mac - Apple Support