If your system files are corrupted, built-in Windows 11 components, such as Windows Security, will not be able to run.
When certain prerequisites are not met, the “Enroll Now” option may not appear in Settings → Windows Update on Windows 10.
You can choose to upgrade to Windows 11 for free if your computer is compatible. You can buy a new PC that already has Windows 11 pre-installed (or opt for an alternative, like a Mac or a Chromebook).