"avr/io.h: No such file or directory" while compiling for board Generic STM32F103C series
More specifically, for a variety of historical reasons, if FastLED can't determine the hardware platform/board being built for, it falls back to avr - and this particular STM chipset isn't directly supported in FastLED at the moment - there's a fork running around that has added support for it, but I haven't had a chance to pull it back into the main library.