Thank you Amey for your patience. 🙂

I’m afraid we can’t reproduce this issue on our demo.

For better debugging purposes, please activate logging on your website, just turn WP_DEBUG to true, like following:
define ('WP_DEBUG', TRUE);
You will find this constant in your wp-config.php file.

More on this in the following topic: https://codex.wordpress.org/Debugging_in_WordPress

I suggest you to make a backup of your website. Then, please deactivate all plugins, activate a default WP theme (i.e. Twenty Seventeen) and then try to run the WP Importer with the following file:
https://raw.githubusercontent.com/WPTRT/theme-unit-test/master/themeunittestdata.wordpress.xml

This may confirm that the problem is related with your configuration and it will give you the necessary error logs to transfer to your server hosting support. Don’t hesitate to share this logs with us, we’ll take a look at them. 🙂

Finally, please also check the WordPress prerequisites here: https://wordpress.org/about/requirements/.

For better performance (and better security) we obviously recommend the version 7.2 for PHP. Nevertheless, if this is not possible for you (server type, incompatibilities with plugins, etc.) we recommend at least version 5.6.X.

We hope this will help.

We remain available for all support requests related with the use of your theme.

Best,