About Find My

Go to Settings, tap on your name, then Find My, tap on Find My iPhone and turn it on. If it is on the email was a scam. If it is off your Apple ID has been hacked; see→If you think your Apple ID has been compromised - Apple Support

Note that Find My can only be turned off by someone who knows your Apple ID and password.

Beware of mail messages and iMessage messages and phone calls, as phishing and scams are common, and sending email addresses, and sending and calling telephone numbers can be spoofed. For details, see >

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

For enabling Find MY (on by default, unless disabled), see >

Set up Find My on your iPhone, iPad, iPod touch, or Mac - Apple Support

You need to have the phone on your person to enable Find My. to enable Find My.

Go to:

Settings > Your Username > Find My > Find My iPhone

Find My must have been turned off previously by someone with access to your Apple ID. If this wasn't you, you should change your password as it is likely someone has unauthorised access to your Apple ID.

If you think your Apple ID has been compromised – Apple Support