The OnStar free Basic Plan should allow you to use either app (the older OnStar Remote or the newer Mychevrolet app) for the basic remote functions such as remote start/stop and remote lock/unlock, but to access the Vehicle Status subsection of the app to see the charge status, total/ev miles, tire pressure and gas level, you need a paid OnStar plan.
The type of OnStar plan you get "free" so to speak when you purchase a Volt has varied over the years from a more full service type that expires after a trial period to the type that only provides the remote start/lock functions for a few years. If you can log onto the OnStar.com website you’ll see information on your current plan, or you could push the OnStar blue button and ask if your current plan is more than the simple basic plan where the app only allows use of key-fob-type functions (and perhaps get help with password resets - your user name is likely your full e-mail address, including the part following the @).
Much of your stats data can also be viewed by logging onto the www.myvolt.com
websites (I viewed mine on both sites today). Those sites remain active to you with any active OnStar account, including the free Basic Plan (also applies to a Volt registered on voltstats.net). Note: your choice of browser may affect your success in viewing data on the myvolt and mychevrolet sites. Firefox sometimes works better for me than Internet Explorer.