I believe something went wrong while doing the setup of the SDK and the editor your using couldn't locate the Qlik.Engine package.
Try following the steps from the following link:
So after installing via nuget, there is a new folder created in the solution called 'packages'.
1. open windows explorer folder
2. Navigate to the packages via folder window: ...packages\QlikSense.NetSDK.126.96.36.199\lib\net452
3. Copy and paste the files into the bin folder.