Make sure that you already have added a delivery method Cash on delivery
Go to Settings > Payments > Click on + Payment method.
Add the custom name: Cash on delivery.
Change also the message template. This message is displayed when the order is placed.
Remove the info, for example, about your bank account number and add a few sentences related to the way the customer can pay when the order is delivered.
Finally, connect this type of payment with the appropriate delivery method – i.e. cash on delivery.