Generating reports with the Solver App is done with the "Reports" button along the top of the App, between the Reset and Solve buttons. You must first run a Solve to produce the Answer and Sensitivity report.
For a Sensitivity report, the following conditions for the model must be met, otherwise you will only receive an Answer Report:
1) A solution must be found
2) A linear or non-linear engine must be used to solve
3) There can be no integer constraints within the model