Al-Salam Alaykom Yousef,
Thanks for purchasing Circle Flip and sorry for any inconvenience. We will make sure that you have a satisfying experience with our theme.
Please check the php_memory limit. That have been the issue with some having the same trouble.
Kindly contact your hosting providers and ask to update php_memory_limit to 256 (even temporarily for you to install the demo)
Please let me know if that doesn’t work, and include admin access to the installation if possible.