Check the QT5 -> Qt Options dialog and ensure that the path to your Qt installation ends at the directory that contains the "bin", "doc", "include", "lib"... subdirectories and be sure there is a default installation selected in the combobox. In my case, for the 32-bit version of Qt for Windows and MSVC 2015, this is "<path to root of installation>/5.11.1/msvc2015". I have multiple versions of Qt installed.

If you didn't actually install Qt (eg. using Qt's online or self-contained installer) then you should probably back up and do that, because the Qt add-in depends on a specific directory structure for Qt components.