You can implement Cash on Delivery using Commerce Offline Payments.

To implement, download and install the Custom Offline Payments module. The follow the directions below.

1. Navigate to Administration → Store → Configuration → Custom Offline Payment → Add payment method.
2. For Title, enter "Cash on Delivery".
3. Under Description, enter text to be shown to the customer during checkout.
4. Ensure that Status is checked.
5. Save the form

Now, you'll have a Cash on Delivery payment option during checkout.