Buy one get second 50% off for WooCommerce product variations

To get a 50% Off on the 2nd item for some specific product variations of a variable product, you will use the following instead:

add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );function add_custom_discount_2nd_at_50( $cart ){    if ( is_admin() && ! defined( 'DOING_AJAX' ) )        return;    // YOUR SETTINGS:    $product_variations_ids = array(1572, 1573); // <== HERE your targeted product variations    // Initializing variables    $discount = 0;    $product_names = array();    // Loop through cart items    foreach ( $cart->get_cart() as $key => $cart_item ) {        if ( in_array( $cart_item['variation_id'], $product_variations_ids ) ) {            $qty   = (int)    $cart_item['quantity'];            $price = (float)  $cart_item['data']->get_price();            $name  = (string) $cart_item['data']->get_name();            if ( $qty > 1 ) {                $discount -= number_format( $price / 2, 2 );            }            elseif( $qty = 1 ) {                $product_names[] = $name;            }        }    }    // Applying the discount    if( $discount != 0 ){        $cart->add_fee('Buy one get one 50% off', $discount );    }        //  Display a custom reminder notice on cart page (otional)    if( ! empty($product_names) ){        wc_clear_notices(); // clear other notices on checkout page.        if( ! is_checkout() ){            wc_add_notice( sprintf(                __( "Add one more to get 50%% off on the 2nd item for %s" ),                '"<strong>' . implode(', ', $product_names) . '</strong>"'            ), 'notice' );        }    }}

Code goes in functions.php file of your active child theme (or active theme) Tested and works.

To get 1 item at 50% OFF for each item purchased, instead of 50% OFF on the 2nd item, replace:

$discount -= number_format( $price / 2, 2 );

by:

$multiplier = ( $qty % 2 ) === 0 ? $qty / 2 : ( $qty - 1 ) / 2;$discount  -= number_format( $price / 2 * $multiplier, 2 );

To get the 2nd one Free instead of 50% OFF on the 2nd item, replace the code line:

$discount -= number_format( $price / 2, 2 );

by:

$discount -= $price;

To get 1 item free for each item purchased, instead of 50% OFF on the 2nd item, replace:

$discount -= number_format( $price / 2, 2 );

by:

$multiplier = ( $qty % 2 ) === 0 ? $qty / 2 : ( $qty - 1 ) / 2;$discount  -= $price * $multiplier;

You were really close!
The discount amount calculation were wrong

add_action('woocommerce_cart_calculate_fees', 'add_custom_discount_2nd_at_50', 10, 1 );function add_custom_discount_2nd_at_50( $wc_cart ){if ( is_admin() && ! defined( 'DOING_AJAX' ) ) return;$discount = 0;$items_prices = array();$qty_notice = 0;  //  <==  Added HERE// Set HERE your targeted variable product ID$targeted_product_id = 1571 ;foreach ( $wc_cart->get_cart() as $key => $cart_item ) {    if( $cart_item['product_id'] == $targeted_product_id ){        $qty = intval( $cart_item['quantity'] );        $qty_notice += intval( $cart_item['quantity'] ); //  <==  Added HERE        for( $i = 0; $i < $qty; $i++ )            $items_prices[] = floatval( $cart_item['data']->get_price());    }}$count_items_prices = count($items_prices);//to get the discount of lowest price sorting in descending order rsort($items_prices);if( $count_items_prices > 1 ) foreach( $items_prices as $key => $price )    if( $key % 2 == 1 ) $discount -= number_format($price / 2, 2 );if( $discount != 0 ){  // The discount  # Note: Last argument in add_fee() method is related to applying the tax or not to the discount (true or false)$wc_cart->add_fee('50% off de segunda almohada' , (($price/2)*($discount))^⁻1, true  ); //EDITED  // Displaying a custom notice (optional)  wc_clear_notices();  if(!is_checkout()){  wc_add_notice( __("Hurrah!! You got 50% off discount on the 2nd item"), 'notice');}}//  Display a custom notice on cart page when quantity is equal to 1.elseif( $qty_notice == 1){     wc_clear_notices();  if(!is_checkout()){    wc_add_notice( __( "Add one more to get 50% off on 2nd item" ), 'notice');}}}