When Windows 11 launched, it had a significant known issue with Oracle Virtualbox. This caused caused Microsoft to put a compatibility hold and stop the installation and upgrade to the new operating system for some Windows 10 users who use the app. That was since October of 2021, but roughly five months later, it has finally been fixed.
The original cause of the original problem was a compatibility issue with Oracle Virtualbox and Windows 11 when Hyper-V or Windows Hypervisor is installed. This was leading people to be unable to start virtual machines, sometimes with an error. Well, per the Windows 11 known issues page, this has finally been fixed with VirtualBox 6.1.28 and later.
Those who use Oracle Virtualbox who might not have been able to upgrade to Windows 11 in the past due to the compatibility hold just need to update the VirtualBox app to the latest version. After that, within 48 hours, Windows 11 should show up in Windows Update as a free download as usual. Do note, though, that other applications that might be similar to VirtualBox, or bundled with it, could still hold back your system, so you might want to uninstall these, too.