FreeRTOS is a real-time OS microkernel used as an OS target by EtcPal. To use EtcPal with FreeRTOS, you must set the ETCPAL_OS_TARGET variable to freertos.

set(ETCPAL_OS_TARGET freertos CACHE STRING "The OS target for EtcPal")

Additionally, you must provide a target called FreeRTOS to help EtcPal find the FreeRTOS headers. If you are not building FreeRTOS from CMake (e.g. if your project is not a CMake project), this can simply be an interface library:

add_library(FreeRTOS INTERFACE)

target_include_directories(FreeRTOS INTERFACE [freertos/include/paths...])

If you are building FreeRTOS with CMake, make sure your FreeRTOs library target is called FreeRTOS exactly and has its include paths set using PUBLIC or INTERFACE.