I have installed libffi on my Linux server as well as correctly set the PKG_CONFIG_PATH environment variable to the correct directory, as pip
libffi
PKG_CONFIG_PATH
pip
You need to install the development package as well.
libffi-dev on Debian/Ubuntu, libffi-devel on Redhat/Centos/Fedora.
libffi-dev
libffi-devel