The model simplification operation requires at least one of the two datatypes:
Zero Value Fluxes - that can be obtained following a structural analysis or performing FVA. Equivalent fluxes.
To simplify a model access the File -> Model Simplification menu:
After this just select the project name, and which datatypes you want to use in the simplification (at least one of them is mandatory).
And that's it! Press OK and a new project will be added to the clipboard!