Seems to me that you are trying to follow the instructions in basix/INSTALL.md at main · FEniCS/basix · GitHub, Advanced -> C++ library, but you missed the very first line
In the cpp/ directory:
(talk about the pot calling the kettle black: I’ve just done the same mistake in Compile Basix C++ backend and python wrappers separately · fem-on-colab/fem-on-colab@496b695 · GitHub)