Error with simple products

Thanks for your input, it was very useful. I’ve contained a slightly wider block of code within the new condition…

/**
 * Check at checkout if any
 * 
 * @access  public
 * @since   1.0.0
 * @param   Array   $fields
 * @param   Object  $errors
 */
public function ycve_variation_validate_dt( $fields, $errors ){

    foreach ( WC()->cart->cart_contents as $key => $item ) {
        
        $product_id = $item['product_id'];
        $product    = wc_get_product( $product_id );
        
        // Check if the product is a variable product
        if ( $product->is_type( 'variable' ) ) {
            $variations = $product->get_children();
            
            if( ! empty( $variations ) ){
                foreach( $variations as $variation ){
                    $this->ycve_check_exp_date_to( $variation );
                }
            }
        
            $newProduct     = wc_get_product( $product_id );
            $newVariations  = $newProduct->get_children();
        
            if ( ! in_array( $item['variation_id'], $newVariations ) ) {
                
                $name       = $item['data']->get_name();
                $itemsCount = count( WC()->cart->get_cart() );
                $cart_url   = wc_get_cart_url();
                $errorMsg   = '<b>' . $name . '</b> it is really not available! already removed from cart!! <a href="' . $cart_url . '"><b>Check cart</b></a>';
            
                if( $itemsCount > 1 ){
                    $errorMsg = 'Product: <b>' . $name . '</b> in your cart, it is really not available and already removed from cart!! <a href="' . $cart_url . '"><b>Check cart</b></a>';
                }
            
                $errors->add( 'validation', $errorMsg );
            }
        }
    }//cart end
}