How to resolve vue js and firebase error?

Question

firebase

[Vue warn]: Property or method "selected" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

found in

---> <Account> at src/components/User/Account.vue
       <VContent>
         <VApp>
           <App> at src/App.vue
             <Root>