Need to know a little bit more about your configuration to say for sure.
Double check your DNS entries. Is your App domain point back to your SharePoint server?
in DNS you need your root record for the app domain to have a CNAME entry that points to a web app on your SharePoint server.
On the SharePoint side - the Web App must not have host headers configured in IIS. If you have host headers configured you need to either remove them or you need to extend the web app with a new address that you can point your App domain too - again without host headers.
If you are only using local hosts file, when you deploy the app via Visual Studio it should create an entry that will allow you to test your app based on the isolated app domain you created.