Cannot install JEWEL in ubuntu 20.04

I would recommend to use packages from Ubuntu repositories.
The 18.04 LTS have LHPADF packages. We can download them to Ubuntu 20.04 LTS to install:

cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/universe/l/lhapdf/liblhapdf0v5_5.9.1-6_amd64.deb
wget http://archive.ubuntu.com/ubuntu/pool/universe/l/lhapdf/liblhapdf-dev_5.9.1-6_amd64.deb

sudo apt-get install ./liblhapdf0v5_5.9.1-6_amd64.deb ./liblhapdf-dev_5.9.1-6_amd64.deb

and then compile JEWEL:

sudo apt-get install gfortran build-essential

cd ~/Downloads
wget https://jewel.hepforge.org/downloads/?f=jewel-2.2.0.tar.gz -O jewel-2.2.0.tar.gz
tar xvzf jewel-2.2.0.tar.gz
cd jewel-2.2.0/
make

to use it by ./jewel-2.2.0-vac or ./jewel-2.2.0-simple.