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/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component" xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in overrided checkout_cart_index.xml file of module-checkout in your theme.

Now you can see discount there.

/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component" xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in checkout_cart_index.xml file of Magento_checkout in your theme by Overriding it there.

Now you can see discount there.

/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component" xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in overrided checkout_cart_index.xml file in your theme.

Now you can see discount there.

/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component" xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in overrided checkout_cart_index.xml file of module-checkout in your theme.

Now you can see discount there.

/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component"  xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in overrided checkout_cart_index.xml file of module-checkout.

Now you can see discount their.

/vendor/magento/module-sales-rule/view/frontend/layout/checkout_cart_index.xml

Take below code from above file

<item name="discount" xsi:type="array"> <item name="component" xsi:type="string">Magento_SalesRule/js/view/cart/totals/discount</item> <item name="config" xsi:type="array"> <item name="title" xsi:type="string" translate="true">Discount</item> </item> </item> 

Insert it in overrided checkout_cart_index.xml file in your theme.

Now you can see discount there.