One of the requirements to run Windows 11 is your PC is must be equipped with a TPM (Trusted Platform Module) 2.0 security chip, and it also has to be enabled in the BIOS.

Source: https://hothardware.com/news/registry-hack-remove-windows-11s-watermark-unsupported-pc

TPM 2.0 is a security feature that needs to be enabled on PCs in order for users to upgrade to Windows 11. However, numerous older PCs do not support TPM 2.0, and even on many newer desktops or laptops, this feature is not enabled by default.

Source: https://www.crn.com/news/computing/windows-11-adoption-slowed-down-in-may

TPM 2.0 is a security feature that needs to be enabled on PCs in order for users to upgrade to Windows 11. However, numerous older PCs do not support TPM 2.0, and even on many newer desktops or laptops, this feature is not enabled by default.

Source: https://www.crn.com/news/computing/windows-11-adoption-slowed-down-in-may

While there this, requiring a TPM chip also caused updates to fail on systems that had one.

Source: https://betanews.com/2023/07/13/microsoft-is-testing-a-windows-11-tpm-troubleshooter/