What do I need to upgrade to make this PC compatible with Windows 11?

The B550 Phantom Gaming 4 should support both Secure Boot and TPM 2.0, so the motherboard probably isn’t the part that needs replacing. The missing detail is your CPU model. Older Ryzen processors may not meet Windows 11’s supported CPU list even though they work with the B550 board. Check the exact processor model and compare it with Microsoft’s supported list before buying anything.

If the CPU is too old for official Windows 11 support, a newer compatible Ryzen processor may be the only hardware change needed; the B550 board supports a broad range of Ryzen CPUs, subject to the installed BIOS version. Tools that create installation media with requirement checks disabled can bypass some Windows 11 restrictions, but that is not the same as making the PC officially supported and may create update or security limitations.

Also check the motherboard BIOS version. Later BIOS releases for this board added Windows 11-related support and may set the needed defaults. Update using ASRock’s instructions, then confirm that CSM is disabled, UEFI boot is enabled, AMD fTPM is turned on, and Secure Boot is active. You can usually keep the motherboard, RAM, storage, graphics card, and power supply unless one of them has a separate compatibility or performance issue.

First check the BIOS rather than replacing hardware. In Windows, open System Information and look at BIOS Mode and Secure Boot State. In the firmware settings, enable AMD fTPM and Secure Boot. If BIOS Mode says Legacy or CSM, the Windows installation may need to be converted from MBR to GPT before switching to UEFI-only mode. Back up important files first, since changing boot settings incorrectly can make Windows temporarily unbootable.