In Qt-Creator 3.6.0, selecting Help -> Contents from menu, the following error
message is displayed:

The page could not be found
Error loading: qthelp://org.qt-project.qtcreator.360/doc/index.html
Check that you have the corresponding documentation set installed.

Probably the documentation it's not installed into the default folder and
Qt-Creator configuration was not updated to point to the right location.

The fix which worked for me was to select Tools -> Options... -> Help ->
Documentation -> Add, and select the following file:

/usr/share/doc/packages/libqt5/qtcreator.qch