how long have you had the car? Was it working before and stopped? If it never worked, have you established an account and added the vehicle, dealer should have helped you with this during delivery.
Occasionally there have been reports of it not working at the end of your OnStar trial period, if that is the case make sure the car is registered when you log into the app or the web based account and shows on your vehicle list. If it does you may need to uninstall and reinstall the app.
If none of that works you need to contact OnStar and tell them that you are supposed to have the "EV Mobile Command" service free with the vehicle while onstar Basic is active, free with the EV for 5 years. do a search on this site and you can find others that had this issue and got it resolved... Not every OnStar rep is aware of this and some give incorrect information that you "must have data or a premium service active"
This is not the case. With the Bolt or the Volt all you need is OnStar Basic (free) and "ev mobile command" should and must be activated