My iPhone 14 pro has been stolen and I can’t find it on “Find My” app!

Assuming that you set up the Find My feature on the phone, it will show up on the Find My map if the following conditions exist:

1) The phone has good battery power....and....

2) The phone has an Internet connection where it is located

This is the only way that a lost / stolen phone can be tracked, since Apple does not get involved with lost or stolen phones.

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

no the serial number is of no use here

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

the police can in theory track it by getting a warrant and forcing the carriers to track it by the IMEI number

but I think they only really pull out those type of guns for missing persons cases