APK for Android cannot be built

Hi there,

I have been working on an AR app recently using Vuforia and Unity 20192.14f. It is about markerless tracking using ground plane. The preferences refer to SDK, NDK and JDK as installed with Unity. For some reason, I am not able to build the app as it won't be saved. Nothing happens if I click on "Save" - no errors either in the Console or the Editor log. The same when I try to build a blank APK. Any ideas what I should check?

Best regards Eva