Qt Creator needs a compiler set up to build. Configure a compiler in the kit options

i was also having the same problem so what i did is

For linux

sudo apt-get install g++ 
sudo apt-get install libgl1-mesa-dev libglu1-mesa-dev

and then closed the qt creator and restarted it and it worked

i do not know which of the above two made it work but it worked !!

For Windows

Below given solution have worked for few people. so it is worth a try.

Delete this file
C:\Users\AppData\Roaming\QtProject\qtcreator\toolchains.xml
to solve the problem.