Hello,
Boldgrid uses php-curl to retrieve themes, If this is not enabled you would get that error, I would recommend checking with your provider to ensure it is enabled on your server, I would also ensure the permissions are correct in your webroot so it can write the files after retrieving them. if that does not solve your issues i would recommend reviewing any error messages in your PHP error log and your web servers error log and posting them here so we can help you troubleshoot them.
Best Regards,
KyleM