Setting API script is not showing in submenu

I cannot recognize where is my problem. Anyone can check my script https://github.com/devrashed/devrashed/tree/main/themex%20plugin how to fix it.

setting_page.php

class settings_page
{
    public function setup_sections() {
        add_settings_section( 'our_first_section', 'My First Section Title', array( $this, 'section_callback' ), 'smashing_fields' );
        add_settings_section( 'our_second_section', 'My Second Section Title', array( $this, 'section_callback' ), 'smashing_fields' );
        add_settings_section( 'our_third_section', 'My Third Section Title', array( $this, 'section_callback' ), 'smashing_fields' );
    }

    public function section_callback( $arguments ) {
        switch ($arguments['id'] ){
            case 'our_first_section':
                echo 'This is the first description here!';
                break;

            case 'our_second_section':
                echo 'This one is number two';
                break;

            case 'our_third_section':
                echo 'Third time is the charm!';
                break;
        }
    }


    public function setup_fields() {
        add_settings_field( 'our_first_field', 'Field Name', array( $this, 'field_callback' ), 'smashing_fields', 'our_first_section' );
    }

    public function field_callback( $arguments ) {
        echo '<input name="our_first_field" id="our_first_field" type="text" value="' . get_option( 'our_first_field' ) . '" />';
        register_setting( 'smashing_fields', 'our_first_field' );
    }
} /*END the setting class*/

?>