The module Qt3DQuick has a transitive dependency to 3dcore_private. This leads to "Project ERROR: Unknown module(s) in QT: 3dcore-private" although private headers are not needed in a project. Ubuntu does not provide private headers of 3dcore, thus, you cannot build projects based on Qt3DQuick on Ubuntu.

I helped myself by defining 3dcore_private:

QT.3dcore_private.name = Qt3DCore
QT += 3dquick

Since this workaround runs fine dependency to 3dcore_private is unnessesary.