Why is the Command-Tab not working on my MacBook Pro?

Command - Tab still works for app switching even with MacOS 15. My first recommendation would be to update Sonoma. Even if your Mac cannot run Sequoia (OS 15), the latest version of Sonoma is 14.7.4.

• Does the problem remain after a restart?
• Did you install any third party software such as Alt-Tab or Karabiner that changes the default behavior?

It may help to identify and third party processes on your Mac that may interfere with the basic functionality of app switching by posting the free EtreCheck report. It contains no personal information.

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I also use Command-tab all the time, and it continues to work as always.

Did you restart your Mac?

Also, while it probably does not explain it: the latest version of Sonoma is 14.7.4, many many updates and security fixes since 14.1.2. If you can't or don't want to upgrade to Sequoia, then at least you should update your Sonoma to the latest version.