SU uses the domain “google.com” (sigh) for checking the availablity of an internet connection and therefore fails if “google.com” is blocked by your firewall or proxy.
If valid open with a text editor as e.g. Windows Notepad (not word processor) the file “C:\Program Files\SketchUp\SketchUp 2018\resources<locale>\i18n.dat”. Go to section “[ONLINE CHECK]” and change value of line “ONLINE_TEST_URL=” from the default URI “Google” to e.g. your own domain or “http://help.sketchup.com”.