Double check your manifest, your first activity should have tag
<intent-filter>
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
inside of activity tag.
If that doesn't work, look for target build, which located in the left of run button (green-colored play button), it should be targeting "app" folder, not a particular activity. if it doesn't targeting "app", just click it and choose "app" from drop down list.
Hope it helps!