https://www.techradar.com/sg/how-to/macos-11-big-sur-problems-how-to-fix-the-most-common-issues
How to fix macOS Big Sur install problems
If your macOS Big Sur problem is in installing the operating system, first make sure you check out our how to download and install macOS Big Sur guide, which will take you through the steps you need to take to safely install the new operating system.
First of all, you'll want to make sure you have a Mac that can run macOS Catalina. The following Macs are compatible:
12-inch MacBook (2015 and later)
MacBook Air (2013 and later)
MacBook Pro (Late 2013 and later)
Mac mini (2014 and later)
iMac (2014 and later)
iMac Pro (all models)
Mac Pro (2013 and later)
When you have a compatible Mac and the macOS Big Sur download completes, but there’s no prompt for the installation to continue, then search in your Mac’s application folder for a file called 'Install macOS 11'. Double-clicking the file should begin installation.
If you encounter the macOS Big Sur problem where the installation fails due to low disk space, simply restart your Mac and press Control + R while it’s booting to enter the Recovery mode. Select ‘Disk boot’ to boot normally, then remove unnecessary files to make space.
macOS Big Sur is stuck on 'Setting Up Your Mac'
If you have installed macOS Big Sur, but then your Mac gets stuck on the 'Setting Up Your Mac' message, you'll need to force a reboot of your Mac or MacBook. Don't worry, the 'Setting Up Your Mac' message means the update is finished, so you won't lose any data or damage anything by restarting your Mac.
To do so, press and hold down the power button for a few seconds until the Mac or MacBook turns off. Wait a few more seconds, then turn it back on. Your Mac or MacBook should now boot into macOS Big Sur successfully.