This functionality can be only applied to BASIC scenarios' sheet created with the scenarios wizard.
If you want to apply a Goal Seek analysis directly on the model use the Goal Seek tool integrated in the Model Analyzer for Excel

Follow these steps to use it:
1) Create a BASIC scenarios' sheet with the Scenarios' Sheet Wizard.
2) Open the Set Scenario in Goal Seeker Analysis dialog.
Define the input and outpu variables you want to work with and finally click the Run goal seek button (with a Gear icon on it)
3) This action runs the Goal Seek analysis, and at the same time setting of variables is saved in a hidden sheet in the same book containing your model. DO

NOT delete them or the tool will no longer work.
esta configuracion se guarda en una hoja oculta en el mismo libro donde esta su modelo, no los elimine
de lo contrario no funcionará esta herramienta.