Magento 2 has native reorder functionality to enable or disable, allow reorder at the store level, from store Configuration setting.
By Default Reorder is Enable for every Order.
You can manage reOrder at the store view level. You can allow reorder functionality in one store and disable it to the second store of the same website.
You can see the ReOrder link from Registered customer Dashboard, My Orders Tab.
Click on My Orders Tab, You can see a list of Previously placed order items with last(action) column contains reorder link.
Log in with admin panel,
Click Left navigation Stores -> Configuration -> Sales -> Sales
Click on Reorder tab,
Choose Yes/No from Allow reorder field.
When complete, click Save Config.
Reorder from Admin User via Admin panel:
1. You can also reorder from Admin Panel with admin credential, Go to Customers -> Find Specific Customer
Click on Orders Tab, Click on reorder link from list of orders of customer from order grid.
2. Go To Order you want to reorder.
Click on Sales -> Orders,
Edit Specific Order from Order grid,
Click on Reorder from top button in order view page.