Sorry to hear of the problem you are having.
It seems your site or server is prohibiting theme to import demo content from github using cURL.
In this case we can install demo content manually.
Please tell me which demo content do you want to install?
Also could you please share me your site login details and site FTP or cPanel details so that i can do it for you?
You can share these details privately by adding them in the box having text “Click here to share private content. Only you and forum moderators will bee able see it.”