If you are using online font generators they will break variable fonts as they are designed for static font faces. They will remove all except the default font face.
Create a folder on your desktop. Open your terminal.
Type CD and drag your folder. That way you don’t need to type your path.
Build
git clone --recursive https://github.com/google/woff2.git
cd woff2
make clean all
Run
./woff2_compress path_to_font/myfont.ttf;
You will get a converted .woff2 file next to your file.