Make sure SketchUp is closed. Go to Finder, and from the Go menu, choose Go to Folder…
Copy and paste in this text, then click Go:
/Applications/SketchUp 2018/SketchUp.app/Contents/Resources/Content/Resources/en-US/i18n.dat
The file that gets selected includes a value that is used for when checking to see if you are online or not. The old URL that was put into SketchUp 2018 seems to be failing for some people. A solution that has worked for several people is to edit the URL to be something else.
The file can be opened with TextEdit, you can do that by dragging the file on to TextEdit’s icon in the Dock. When the file is open, look for these lines:
[ONLINE CHECK]
ONLINE_TEST_URL=Google
Change the second line. Later versions of SketchUp used bing.com, so try making those two lines be like this:
[ONLINE CHECK]
ONLINE_TEST_URL=https://www.bing.com
Save that and close the document, then open SketchUp again.
Later operating systems tend to object to you trying to access an insecure web site, and the address that starts with ‘http’, which was perfectly good 3-4 years, can now trigger a security warning. By making it be ‘https’, that should keep the system happy.
Please let me know how that goes.