Order Comments
Order Comments tool enables customers to enter their comments for an order on checkout. You can enable Order Comments from Settings → General → Cart & Checkout. Scroll to the Order Comments section, enable the comments and enter the text to show:
You can make the field required if needed.
Viewing Order Comments
Customer comments for the order are part of the order's details. They are available on theOrders page:
You will also see the customer comments on the order’s details page: