Scroll down and Click on "The Certificate Warnings Issue"
Under the FAQ section right click on the first link and click on "Save Link As"
Change the file name on the top bar from Computerisms.Root.Certificate.pem to Computerisms.Root.Certificate.crt
Save the file on your Desktop(Note that you can save the file anywhere you wish but couple of the next following commands in terminal would not work without changing the path in command, so it is recommended if you save it on your Desktop)
Click on save
Click on the Unity Dash located on the top left corner
Open a terminal by typing "terminal" in unity dash search or press Alt+Ctrl+t