WordPress Fatal error: Call to undefined get_header() in index.php on line 15 [closed]

Something went wrong with your installation. As commented by @Sumit get_header() is a core function and cannot be undefined.

To troubleshoot:
Reinstall WordPress and enable WP_DEBUG in wp_config.php:

define( 'WP_DEBUG', true );

If it is on a live site you might want to use this instead:

define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );

This will list your errors in an error log in the wp-content folder.

Your site should display normally using a default theme with no errors.

Activate your theme and check for errors.