unable to add custom page/tab to my account woocommerce

Try adding this code to your add_action ‘init’ function.

//  Let WooCommerce add a new custom endpoint "earnings" for you
add_filter( 'woocommerce_get_query_vars', function( $vars ) {
    $vars['earnings'] = 'earnings';
    return $vars;
} );

//  Add "earnings" to the WooCommerce account menu
add_filter( 'woocommerce_account_menu_items', function( $items ) {
    $items['earnings'] = __( 'My earnings', 'your-text-domain' );
    return $items;
} );

//  Display your custom title on the new endpoint page
//  Pattern: woocommerce_endpoint_{$your_endpoint}_title
add_filter( 'woocommerce_endpoint_earnings_title', function( $title ) {
    return __( 'My earnings', 'your-text-domain' );
} );

//  Display your custom content on the new endpoint page
//  Pattern: woocommerce_account_{$your_endpoint}_endpoint
add_action( 'woocommerce_account_earnings_endpoint', function() {
    echo __( 'Hello World', 'your-text-domain' );
} );

//  Only for testing, REMOVE afterwards!
flush_rewrite_rules();