Welcome to the Apple Support Communities. I understand you have a question about engraving.
If you're not seeing the option to engrave when you're placing the order, it would not be available for that particular product. You can see which products would be eligible to be engraved here. Engraving and Gift Wrap :
Make it a gift they’ll never forget by adding a personal laser-engraved message to any iPad, iPod touch, or Apple Pencil (2nd generation). After selecting the product you want, you’ll be taken to a page where you can add your personal message.Based on the above, it does not look like you'd be able to order an engraved iPhone. Hopefully this helps to answer your question.
Kind regards.