I was deeply puzzled this afternoon, when I configured my App Catalogue and all was fine - except I was unable to access the SharePoint Store.
"Sorry we can't seem to connect to the SharePoint store"
hmm.. checked network, DNS, proxy, certificates, App/System Logs, ULS logs.. nothing.... then it clicked!
Dive into Central Admin->Manage Web Applications,select the web-application and up in the Ribbon, select Manage Featuresnow ACTIVATE for internet access to the Store!
"Apps that require accessible internet facing endpoints""Allow users to acquire apps that require internet facing""endpoints from the SharePoint Store."
IISRESET and voila! everything works fine.
This information is a bit misleading, I'm afraid. You can access the Store with the Internet-facing endpoints feature switched off, but certain Apps will be greyed out. I experienced your error on a newly configured environment and it seemed to resolve itself
overnight (whether that be due to recycling of application pools or a timer job running, I'm not sure). I have 2 environments configured with Internet-facing endpoints turned off and can access the Store and install Apps just fine. See the bottom of:
http://technet.microsoft.com/en-us/library/fp161236(v=office.15).aspx "The SharePoint Store contains apps for SharePoint intended for use with sites that require Internet-facing endpoints. By
default, these apps are not available (greyed out and cannot be purchased) because they are incompatible with most sites."