Directory of standard AVR header files

I'm running Ubuntu 20.10 and I'm trying to compile c-files from a custom directory (~/Desktop/AVR/.../main.c). I'm writing my c-files in VSCode. All of that works great.

My issue is that VSCode can't detect the include header files, even though I've added the directory where delay.h, io.h, etc. are located, "/usr/lib/avr/include/avr", to the includePath in c_cpp_properties.json. Am I using the wrong directory?

I have all the avr related packages installed (avr-libc, avr-gcc, etc.), so that isn't the issue. Any takes?