Hi,
1. Make sure you have the latest version of the theme and the core plugin.
2. Try disable the xStore Core plugin and try to activate the theme, after activation enable the plugin again.
3. Make sure you’re having all the requirements http://xstore.helpscoutdocs.com/article/60-xstore-theme-requirements
4. Make sure your hosting have cURL enabled, installed and on latest version.
Let us know,
Thanks,
laranz.