How can I enable Visual Look Up feature on my iPhone?

Yes it is. Your iPhone 15 Pro is a supported model and the feature is also region dependent with these languages:

• English
• French
• German
• Italian
• Japanese
• Spanish

Of course the option will only appear when an object has been detected in a photo as seen here:

Use Visual Look Up to identify objects in your photos and videos on iPhone - Apple Support

Go to Settings > Search, and make sure you have Show Related Content turned on. Good point by lobsterghost1, I did not catch that.

Siri MUST be on for Apple Intelligence and it's associated features to work.

If you looked up the link that Mac Jim ID provided and it still isn't working, take a look at this Apple Support YouTube video to see if you are following the directions correctly. You do not indicate what it is that you are looking at and trying to identify.