Icon Daemon is right, but one thing has been missed - do not directly launch the installer after downloading.
To create a Big Sur installer, here is the details:

 1. Search for macOS Big Sur in App Store. (you'll see only "view" if
    you already run macOS 11. You need to click the tab to get into the
    interface, and then see "Get".)
 2. Click on the Get button on the upper right, and the Software Update system preference will open. 
 3. Click the Download button. The download may take a while.
 4. After the download is complete and the installer launches, DO NOT click the Continue button.
 5. Quit the installer by pressing Command-Q.
 6. Go to your Applications folder and you should find an “Install macOS Big Sur” app. You need to copy-paste the installer to your USB drive to create a bootable installer.