Hi @rhasan,
As Open edX is an international product, it is designed to work with names from around the world, which often don’t fall into the simple “First Name” + “Last Name” formula. Here’s a good article from the W3C showing examples of why that doesn’t work.
I believe that the “First Name” + “Last Name” fields you’re seeing exist in the Django admin and in the underlying data model, because they are provided by the framework that Open edX is built on, but Open edX itself doesn’t use those fields. So it’s safe/best to just leave them blank.
My recommendation is to change your business logic so that you only work with full names and don’t need the First Name / Last Name split. Alternatively, you can create your own API to get/set the first_name and last_name fields that exist in the database, but you’ll have to understand that those fields will be ignored by the platform and won’t be used by any of the standard APIs. You can use this extension point to add additional fields to the registration form, including “First name” and “Last name”, if required.