There are 2 possible reasons why this might not be working.
1. A manual install
In this case run the following commands to remove flux
cd /tmp
git clone "https://github.com/xflux-gui/xflux-gui.git"
cd xflux-gui
python download-xflux.py
# EITHER uninstall globally
sudo python setup.py install --record installed.txt
sudo xargs rm -vr < installed.txt
# EXCLUSIVE OR uinstall in your home directory
python setup.py install --user --record installed.txt
xargs rm -vr < installed.txt
2. Missing PPA
There is a possibility that the ppa was removed after flux was installed. You can try adding the repo again and then uninstalling flux and then removing the PPA.
sudo add-apt-repository ppa:nathan-renniewaldock/flux
sudo apt update
sudo apt remove fluxgui
sudo add-apt-repository --remove ppa:nathan-renniewaldock/flux
sudo apt update