CONFIG_ATH79_MACH_AP121 is not available for the default architecture x86.
Result is shown for architecture mips
default or selected kernelversion does not have config value CONFIG_ATH79_MACH_AP121.
Result is shown for kernelversion 5.9.8

Atheros AP121 reference board

modulename: mach-ap121.ko
configname: CONFIG_ATH79_MACH_AP121

Linux Kernel Configuration
└─> Atheros AR71XX/AR724X/AR913X machine selection
└─> Atheros AP121 reference board

Say 'Y' here if you want your kernel to support the
Atheros AP121 reference board.

source code: