No described button for the popup of the variables list.

that is what I found in the doc:
http://doc.qt.io/qtcreator/creator-build-settings.html#using-qt-creator-variables
"...
Select the (Variables) button in a field to select from a list of variables that are available in a particular context.
..."

But there is no any button, see the screenshots.

Here is an old discussion about that, see also the answer there for the additional screenshots and details
http://stackoverflow.com/questions/35970727/use-of-variables-like-builddir-in-qtcreator-kit-settings-in-qt5