From MyChevrolet site, appears you need an Onstar account?
How to Use myChevrolet Mobile App | Vehicle Support | Chevy
Why isn’t my vehicle showing up in the mobile app?
If your correct vehicle isn’t appearing in the app, it is likely an account linking issue. This may be because your account is linked to an old email or you didn’t link your account when you set up your profile. It could also be because you recently made changes to your OnStar account (such as renewing or upgrading, consolidating multiple accounts, etc.) and the changes have not yet been synced. You can try logging out of and back in to the app. If you’re still having trouble, call an OnStar Advisor at (877) 558-8352 for help fixing this issue.