If you encounter this message when trying to access Google File Stream, here are the following steps to fix it.
Click "Open System Preferences", then click "Allow on the bottom right.
Note: If you do not see the "Allow" button in System Preferences, click on the file stream icon on the top and click "Sign In" to get the System Extension Blocked message to appear. This will get the button to show up.
If it does not show up, close System Preferences and open it back up. Then go to "Security & Privacy" > General tab.
After that, click this icon 
on the top of your screen, and click sign in.
It should automatically sign you in.