As far as I know, Firewall for Business Cloud is not offered on Windows Server, either because of technical limitations, or to force you to buy a more expensive product
Actually Avast does support servers. Go to
https://www.avast.com/en-us/avast-for-businessUnder servers, it lists:
It will work with any operating system listed below:
Microsoft Windows Server 2003 and 2003 R2 (any edition with the latest Service Pack, 32-bit or 64-bit, incl. Small Business Server)
Microsoft Windows Server 2008 and 2008 R2 (any edition with the latest Service Pack, 32-bit or 64-bit, incl. Small Business Server, except Server Core)
Microsoft Windows Small Business Server 2011 (any edition with the latest Service Pack, x64bit)
Microsoft Windows Server 2012 and Server 2012 R2 (any edition with the latest Service Pack, x64bit, incl. Small Business Server/Essentials, except Server Core)
Microsoft SharePoint Server 2003/2007/2010/2013
Microsoft Exchange Server 2003/2007/2010/2013. Avast antispam is available for Microsoft Exchange Server 2007/2010/2013