exactly as I suspected, tour getting a .bsp error. Essentially what it means is the file is to big for you to download and it can be fixed by changing some settings via console. I'll post here with the commands as I need to close window to find them, stupid phone.
Ok in console type net_maxfilesize 99 and sv_allowdownload 1 and cl_allowdownload 1
If these commands don't work, the problem lies out side of css. Steam uses ie for Its defualt browser for downloads. So if you have it set to offline mode, that would pose problems when trying to dl maps via the server. Try the following fix
. Make sure you have Internet Explorer on your computer.
2. Make sure Internet Explorer is your default browser. (Open IE > Tools > Internet Options > Programs > Click "make default".)
3. While you are still in the Internet Options box click the Connections tab.
4. Click LAN settings/proxy settings.
5. Click "Automatically detect settings