Can you track a stolen phone using an imei number?

No.

Also see the information in If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support including the following:

If your missing device doesn't appear in the Find My app or at iCloud.com/find, you didn't turn on Find My. There's no other Apple service that can find, track, or flag your device for you. But there are steps that you can take to help protect your data.

Did you report the theft to the Police. If so, the imei number might be proof that the device is yours if it is ever recovered. Track the phone with that number, no.