Windows 11 requires a TPM. The X299 motherboard has TPM in BIOS (fTPM) which was not enabled when the system was built. It can be enabled in the motherboard BIOS' Advanced Menu/PCH-FW Configuration area. In order to install and run Windows 11, your BIOS needs to be set up for UEFI operation (BIOS' Boot Menu/CSM/Enabled). The OS drive also needs to be formatted GPT, not MBR.
Microsoft didn't make the migration very easy. It's probably best to perform a fresh install instead of trying to upgrade, but that's up to you. If you wanted to try upgrading your current Windows 10, you can perform the following:
Preparing Windows 10 for upgrading to Windows 11 on an X299 motherboard when formatted as MBR.
1. Back up your OS if you don't want to lose data. Microsoft updates and upgrades are not always successful, sometimes due to the state of your current OS. Appendix A of the FRED Manual (https://digitalintelligence.com/files/FRED_Manual.pdf) has instructions on how to use our restore media to create an image backup of your OS.
2. Create Windows 11 Installation Media (https://www.microsoft.com/software-download/windows11).
3. Update your motherboard BIOS. The latest has better support for Windows 11 and is available from ASUS (https://www.asus.com/us/motherboards-components/motherboards/workstation/ws-x299-sage/helpdesk_bios?model2Name=WS-X299-SAGE).
4. Open Disk Management and identify the OS drive's letter and disk number (probably C: and 0). Remember to change the details in the commands below, if necessary.
5. In Windows 10, open a command prompt as Administrator and enter bcdboot C:\Windows /s C:. When this completes, reboot the system.
6. After Windows loads, open a command prompt as Administrator and enter mbr2gpt /validate /disk:0 /allowfFullOS. The validation should complete successfully.
7. Enter mbr2gpt /convert /disk:0 /allowFullOS. Once this completes you should see that the OS drive in Disk Management now has an added EFI System Partition.
8. Reboot and get into the motherboard BIOS setup by hitting DEL during bootup. Go to the Boot menu and make sure the boot order shows Windows Boot Manager in the first position.
9. Boot back into Windows. The OS is now ready to be upgraded to Windows 11 by launching setup.exe on the install media. The default option is to retain your files and data. When the update completes you should be able to use your Windows 10 license key, which is applied to the back of your system, for activation.
