Magento Commerce, 2.2.x
Allow Reorders

When enabled, reorders can be made directly from the customer account or from the original order in the AdminThe password-protected back office of your store where orders, catalog, content, and configurations are managed.. Reorders are enabled by default.

Customer Reorder Link in Admin

To configure customer reorders:

1. On the Admin sidebar, tap Stores. Then under Settings, choose Configuration.
2. In the panel on the left, under Sales, choose Sales.
3. Expand the Reorder section.
4. Set Allow Reorder to your preference.