Magento 2 Related products add to cart button issue

By default Magento, there is no setting option in Admin for Add to Cart button on the related products block. If we want to custom the related, upsell, crosssel and new product template, we should override this template vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml.

Change $showCart = true; in case - related product.

We can refer here: Related Products template customize

For displaying the Add To Cart button, edit the below-mentioned file

vendor/magento/module-catalog/view/frontend/templates/product/list/items.phtml

$showCart = true;