CONFIG_ARM64_SSBD is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64
default or selected kernelversion does not have config value CONFIG_ARM64_SSBD.
Result is shown for kernelversion 6.9.9

Speculative Store Bypass Disable

modulename: ssbd.ko

configname: CONFIG_ARM64_SSBD

Linux Kernel Configuration
└─>Kernel Features
└─>Speculative Store Bypass Disable
In linux kernel since version 4.2 (release Date: 2015-08-30)  
This enables mitigation of the bypassing of previous stores
by speculative loads.

If unsure, say Y.

source code: