ался

android:screenOrientation="portrait"

а также

setRequestedOrientation (ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

но это сбой приложения, есть ли альтернативный способ для этого, чтобы работать в Android 8.0.0+?

Logcat:

FATAL EXCEPTION: main
  Process: in.ajtech.finX, PID: 15077
  java.lang.RuntimeException: Unable to start activity ComponentInfo{in.ajtech.finX/in.ajtech.finX.CalendarActivity}: java.lang.IllegalStateException: Only fullscreen activities can request orientation
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
      at android.app.ActivityThread.-wrap11(Unknown Source:0)
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
      at android.os.Handler.dispatchMessage(Handler.java:105)
      at android.os.Looper.loop(Looper.java:164)
      at android.app.ActivityThread.main(ActivityThread.java:6541)
      at java.lang.reflect.Method.invoke(Native Method)
      at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
   Caused by: java.lang.IllegalStateException: Only fullscreen activities can request orientation
      at android.os.Parcel.readException(Parcel.java:1950)
      at android.os.Parcel.readException(Parcel.java:1888)
      at android.app.IActivityManager$Stub$Proxy.setRequestedOrientation(IActivityManager.java:5675)
      at android.app.Activity.setRequestedOrientation(Activity.java:5739)
      at in.ajtech.finX.CalendarActivity.onCreate(CalendarActivity.java:55)
      at android.app.Activity.performCreate(Activity.java:6975)
      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892) 
      at android.app.ActivityThread.-wrap11(Unknown Source:0) 
      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593) 
      at android.os.Handler.dispatchMessage(Handler.java:105) 
      at android.os.Looper.loop(Looper.java:164) 
      at android.app.ActivityThread.main(ActivityThread.java:6541) 
      at java.lang.reflect.Method.invoke(Native Method)