Ubuntu 20.04 really needs more time to mature, so unless you are using 20.04 because 18.04 doesn’t run on your hardware, you may find fewer issues using 18.04.
NASA’s OCSSW software uses libgfortran3, so those of us who use the NASA software just need to copy it to /usr/lib. If you don’t want to install the full OCSSW package, you can try the rather long and messy procedure posted in this forum.