The reason that option is grayed out because you have legacy OPROM is enable. Enable "Secure Boot Control" under Boot section - which will then disable "Load Legacy OPROM" and then it will allow you to Enable "Firmware TPM". I was able to replicate the issue at my end.
Trying to be ready for Windows 11, and was trying to see if my Area-51 R5 was compatible. Everything is good except the TPM requirement. In the BIOS I do see the TPM option disabled under Security but the option is greyed out and I cannot enable it? Does anyone know how to do it?
Area-51 R5, enabling TPM in the BIOS