Hi Shrikant,
It is difficult to say without further information and investigation, or complete device debug log. When installing App Center, you want to ensure you use a publicly trusted SSL certificate, its corresponding private key, and associated certificate bundle. The certificate bundle is comprised of the SSL certificate's intermediate CA's certificates up to the parent root CA's certificate.
Also, when installing App Center (the post-installation-script-configuration-page), ensure you are accessing the "configurator" via the publicly accessible FQDN of the server (which should match the SSL certificate's Common Name (CN) field). The "Server name" field in the configurator must be the shortname of the server. For example:
Access the configurator via proper FQDN: http://appcenter.domain.com:8000
In the configurator, the "Server name" field should be automatically populated with the shortname: appcenter
If the FQDN of the server is: appcenter.domain.com, then the SSL certificate must have a CN value of: appcenter.domain.com
From the mobile device, you can simply access: https://appcenter.domain.com, and it will automatically route you to the appropriate download page for our App Center App (ACA)
Hopefully this information helps, and is related to your issue. If you continue to experience issues, I'd recommend opening a technical support case.
Regards,
Andrew