Your PC may not be eligible for an upgrade to Windows 11. You'll have to do some investigation of your system to determine if it's worthy of the upgrade.
So, on your PC, click once on the Start button and type in: system information and then press Enter. A few items down the list is Processor. Write down the processor value. It will be something like "Intel..." or "AMD...". You need that processor info to check eligibility.
Open a tab in your browser and enter https://github.com/rcmaehl/WhyNotWin11 in the URL field at the top of your browser window and press Enter. Scroll down to the Download section and click on 'Download latest stable release' and run the download.
Let me know if you need further assistance.