Have you configured "Redirect" action to "Tab bar"? Most probably that is the reason for tab bar not loading in preview app. If you have a Splash screen in your app configure "Redirect" action with "Tab Bar" selected from "Select Screen" dropdown, in MAIN_VIEW (control in Splash Screen) >> Time Completed event.
or if you do not have splash screen and want to show Tab Bar on load of app, you need to set "Default Page" in "MISCELLANEOUS SETTINGS" to "Tab Bar"
Now re download your app in preview to check if tab bar works