Two days tyring to figure this out.

CMake Error at CMakeLists.txt:40 (FetchContent_Declare):
Unknown CMake command "FetchContent_Declare".

This must be absolutely vital to the whole build becuse the few lines are preventing the build. Quick question, why isn't this listed as dependancy? Where is it?

Start at CMakeLists.txt line 40 and inspect the surrounding FetchContent_Declare call and build setup. Reproduce the reported CMake configuration error, then determine where that command is provided and what setup is needed for configuration to complete.

Written by the indexing model from the issue text.