App Store Does Not Work - macOs Sierra
TLDR: If you can browse the web with Firefox or Chrome, but not with Safari, then adjusting your network settings might fix both Safari and the App Store.  Start by looking at proxy settings.

I'm hoping you found a solution by now.  Just in case this helps anyone, I'm going to share what worked for me.  I had no problem browsing the web with Firefox or Chrome, but the App Store just showed a grey window like the screenshot in the original post.

During troubleshooting, I discovered that Safari couldn't connect to any web servers.  This suggested that the problem had something to do with the Mac's network settings.  I opened the network settings, selected my network adapter, clicked Advanced, went to the Proxies tab, and unchecked Automatic Proxy Configuration.  That fixed everything.