How can I turn on the find my if the phone is lost

1. Open the Find My app and choose the Devices tab or the Items tab.
2. Select your missing device or item.
3. Scroll down to Mark As Lost or Lost Mode and select Activate or Enable.
4. Follow the onscreen steps if you want your contact information to display on your missing device or item, or if you want to enter a custom message asking the finder of your missing device to contact you.
5. Select Activate.

See this link for further reference If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

• If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support

//support.apple.com/en-us/HT211207

There are certain limits if not enabled, short of recognized fact it's lost..

You can't turn on Find my iPhone-- after you lost your phone ... Find my iPhone should have been turned on-- when the phone was in your possession in iCloud-- when you sighed in-- if you did then-- you can track (locate) the phone Lock or Erase-- go to iCloud.com/find log in with your Apple ID and password your device should show up on the map.

If FindMy was not enabled before you lost it, you can't.