pkg-config indicates libxml2 not found

Solution 1:

You have to ignore the suggestion from pgModeler documentation.

Correct name of the libxml2 configuration file for pkg-config is libxml-2.0.pc, you have already installed it from libxml2-dev package.

Simply proceed with the compilation:

sudo apt-get install git build-essential libxml2-dev qt5-default libpqxx-dev libpq-dev libqt5svg5-dev qttools5-dev

cd ~/Downloads
git clone https://github.com/pgmodeler/pgmodeler.git -b master
cd pgmodeler
git clone https://github.com/pgmodeler/plugins -b master
qmake PREFIX+=/usr/local -r pgmodeler.pro
make -j$(nproc)
sudo make install

Also you should know that pgModeler 0.9.2 is packaged in Ubuntu 20.04 LTS at https://packages.ubuntu.com/focal/pgmodeler, so it installable as simple as

sudo add-apt-repository universe
sudo apt-get update
sudo apt-get install pgmodeler