Magento 2 Related Products Add to Cart Button Issue – Fix

Replace $this->getAddToCartUrl($_item) with $this->getAddToCartUrl($_link). This should work unless you changed variable names in upsell.phtml

You are using Mage class, which has been removed in Magento 2. The Magento 2 equivalent of

<?php echo Mage::helper('checkout/cart')->getAddUrl($product) ?>

is as below:

Method 1

<?php echo $objectManager->get('Magento\Checkout\Helper\Cart')->getAddUrl($product) ?>

Method 2:

Also, Note that you should avoid using Object Manager in Magento 2. It is not a good practice. So, to avoid using Object Manager you have to write following in your template's Block File:

protected $_cartHelper;

public function __construct(
   \Magento\Checkout\Helper\Cart $cartHelper
) {
   $this->_cartHelper = $cartHelper;
}

public function getAddToCartUrl($product, $additional = []) {
   return $this->_cartHelper->getAddUrl($product, $additional);
}

For method 2, you can use Magento\Catalog\Block\Product\View class's getAddToCartUrl function as your reference.