Cant buy new, download or update existing purchased free apps from cloud via iTunes 12.3.0.4?
The help article has several options - these are the ones you can check:


Set the date/time on your PC to match an internet time server
Check anti-virus / firewall software on the PC
Sign out of the app store on iTunes and then sign back in


Barring that, you might need to get at the debug or error logs for the App Store - http://www.apple.com/support/itunes/store/ or AppleCare can help. Maybe someone else can edit in where the logs are on a PC.