I have not run into any problems either creating a new C++ project with the binary 4.9.1 Engine, nor with adding code to a Blueprint project. I have a few things for you to try.
First, in the Library section of your Launcher, please verify your 4.9.1 installation (this may take a few minutes).
If no problems occur with the verification (I don’t expect any), please download one (or both) of [these][2] [projects][3]. Unzip the project and right-click on the .uproject file. Select the “Switch Unreal Engine version” option, make sure it is set to 4.9, then click OK. Please let me know if you see the same error as before.