Android Oreo (API 26) - Create dir in external storage

I have no problems running your existing code on a Nexus 5X running Android 8.0. Using adb shell ls /storage/emulated/0, I see Chores/, and inside there I see Processed Audio/. This is for an app with WRITE_EXTERNAL_STORAGE permission, including runtime permissions.

That being said, ideally, do not use string concatenation to create File objects. Instead, use:

final File dir = new File(new File(Environment.getExternalStorageDirectory(), "Chords"), "Processed Audio");