this folder pointed to files moc_mainwindow.cpp and moc_mainwindow.h, which unfortunately do not exist...
these file is required if you have Q_OBJECT micro in your class. means you using signal and slot.
for creating these file you need to compile mainwindow.h

Do the following steps:
1. right clickon mainWindow.h in explorer tree branch, then select properties.
in properties window goto Custom Build Step-->General then enter in
Command line field : "%qtdir%\bin\moc.exe" "$(InputDir)$(InputName).h" -o "$(InputDir)moc_$(InputName).cpp"

note: here qtdir is environment variable which have path of Qt or you can write Path of Qt directly.

output field: $(InputDir)moc_$(InputName).cpp

2. right click on mainWindow.h then select compile. this process will generate moc_mainwindow.cpp and moc_window.h

3. you add this in "Generated " tree branch.

now you can build your application.