Hi there!
To download this file, the boards manager tries to access the internet. If you have any proxies between your computer and the internet, they may prevent complete communication. For example, at my work we have a proxy and I needed to download some boards and I have the same error.
If you can, try to find a computer that can access the internet normally without any hinderances and download the IDE. Then try to download the board files onto that computer. After that, you can find the directory location of those files on the second computer, transfer them to a flash drive, and then transfer from the flash drive to the same directory on the first computer that cannot access the internet completely. Do this while the IDE is closed, and when you reopen the IDE the board option should be present when you go to Tools->Board.
Let me know if your computer has a proxy in it and what board file you are trying to get, then I can give you more detailed instructions on how to get the files transferred.