Lightshot is a wonderful little utility that enables easy selective screenshots and quick annotations.
However sometimes the Lightshot application can't register the Print Screen key as it's short key as another program has already registered it. The culprit is usually Windows' OneDrive.
To prevent this clashing you need to disable the 'Auto Save Screenshots' feature of OneDrive.
To do so open OneDrive then
Settings -> Auto-save and ensure
Automatically save screenshots I capture to OneDrive is not checked.
You'll need to restart Lightshot to allow it to successfully register the shortkey.