I encountered a problem when creating a project in Android Studio. (Version 1.5.1)
I describe my actions step-by-step:
I had the same issue when I installed android studio 2.1 on ubuntu 14.04.Even the android robot icon which displays the android version used for rendering layout was only showing API Level 24 version and no other versions.
I had to enable API 23 to be downloaded by navigating to the below path
Tools -> SDK Manager -> Android SDK -> SDK Platforms -> Name.
Check the Android 6.0(Marshwallow) option.This will be downloaded now. Now you will be able to see API Level 23 and by selecting which we can get around this issue.