Begin
As a quick workaround to get Flameshot working in legacy mode: bash
mkdir -p "$HOME/.config/flameshot/"
cat > "$HOME/.config/flameshot/flameshot.ini" <<EOF
[General]
useX11LegacyScreenshot=true
EOF
The next time you start Flameshot, a slightly annoying GUI will appear asking you to choose a screenshot method. It’s not perfect, but it’s better than having no screenshot tool at all.