If your mail was sent via USPS First or Standard Class, please note that Lob will not provide a "tracking number". Rather, tracking information (“events”) related to each mail piece will get surfaced via the Lob Dashboard and in the tracking object of the mail piece via the API. This information can be retrieved by doing a GET request on a particular mail piece id. For more information on what each tracking event means, please refer to this page. There is also a possibility your mail was sent out metered, see more information on this here.
If a mail piece was sent with Certified or Registered add-ons and has no tracking number assigned, please reach out to support@lob.com for assistance.