The always on option is only available of the iPhone 14 Pro and iPhone 14 Pro Max, but not to the iPhone 14 model. See the following link to an Apple article: Use Always-On display with your iPhone 14 Pro or iPhone 14 Pro Max - Apple Support
and the following excerpt from that article:
With Always-On display, your iPhone 14 Pro or iPhone 14 Pro Max dims the Lock Screen while still showing helpful information, like the time, widgets, and wallpaper, all while using new technologies that make the display incredibly power efficient. Learn how to use Always-On display.