This error means the game can’t connect to the servers to download any missing in-game data.
We would, first of all, suggest trying the Basic Connection Troubleshooting steps. Should the problem continue then you can you please try the following steps:
Press windows key + R > Type %programdata% > Click OK > Delete the Battle.net and Blizzard Entertainment folders
Press windows key + R > Type %appdata% > Click OK > Delete the Battle.net folder
Press windows key + R > Type %localappdata% > Click OK > Delete the battle.net and Blizzard Entertainment folders
Then make sure to run the Battle.net App as administrator when you restart it (Right click the icon > Run as Admin).
Lastly try flushing your DNS.
If the issue persists, please upload your system files to pastebin.com and reply with the links to the files using the preformatted text option to encapsulate the links.