Xeon E5-1620 is not supported on Windows 11, but you can use PC health check to see if CPU is only incompatible part. How to use the PC Health Check app - Microsoft Supp
If you have TMP1.2 or TPM2.0, UEFI BIOS with Secure Boot and GPT partitioned disk you can manually update Win 10 to Win 11 with these Microsoft instructions:
Ways to install Windows 11 - Microsoft Support
It's also possible to install Windows 11 completely unsupported devices, but you need to find those instructions from net.
NOTE: Microsoft doesn't recommended to install Windows 11 to unsupported devices.