Hi Rubaitul Azad, 🙂
Please note that this issue is related with the customization of a third party plugin, not with the use of your theme. This plugin is compatible with our theme, but not supported. In conclusion, as theme authors, the installation, setting or debugging of third party plugins isn’t part of the support we will provide.
To go further, here is the installation guide from Nextend Social Login and Register plugin:
https://fr.wordpress.org/plugins/nextend-facebook-connect/#installation
We remain available for all support requests related with the use of your theme.
Regards,