Screenshots - All the screenshot shortcut not working in macOS 26.x and happen randomly

Common culprits interfering with the normal screen shot keyboard actions are Keyboard Remapper applications

Examples are Karabiner-Elements, BetterTouchTool, Window managers, Remote desktop software, Screen recording utilities and Clipboard managers

Try booting into Safe Mode and see whether the problem occurs there.

If it does not occur in Safe Mode, a third-party utility is probably intercepting the screenshot shortcuts.

Or, you can follow the previous advise from my colleague and run the Etrecheck application and post the report

Download the Application Etrecheck  ( External Link ) directly from the Developer.

This is a Diagnostic Tool that makes no changes to the computer.

It makes a coherent and readable inventory of both the Hardware and Software used on the computer 

The application is free or paid for added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - Share Report >> Copy , then  paste  >>>> using the Additional Text Icon  <<<<

Do you have any antivirus or "cleaners"?

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.