CONFIG_ARCH_FLATMEM_ENABLE is not available for the selected architecture x86.
Result is shown for architecture loongarch

CONFIG_ARCH_FLATMEM_ENABLE (not configurable)

configname: CONFIG_ARCH_FLATMEM_ENABLE

Linux Kernel Configuration
└─>ARCH_FLATMEM_ENABLE
In linux kernel since version 2.6.13 (release Date: 2005-08-28)
depends
NOT CONFIG_NUMA