<GeoQuiz App> Problem with android:text="@string..." /> not not recognized

Hello everyone,

I am a beginner to the world of android development. I am currently having problem with these lines of code:
android:text="@string/question_text" />
android:text="@string/true_button" />
android:text="@string/false_button" />

The compiler keeps showing error “cannot resolve symbol…” and the preview of the app shows buttons with weird title.
What can I do now ? I try to resolve the problem with recommended suggestion. However, I am not sure about what I am doing.
Please help me with this. Thank you