If you are using a VPN or anti-malware software on your iOS or Android device, you may have some difficulty accessing Yapp. One example of such software is Trend Micro Mobile Security. If you're unsure if you're connecting with a VPN, you should see a "VPN" icon or a key icon in your status bar.
To confirm that the VPN is causing a problem, connect to a trusted wifi or cellular network, disable the VPN in your device settings, and try using Yapp again. If Yapp now works, you can use that as a workaround to initially download an app. Note that much of the content of an app is viewable offline once downloaded!