How to set a partial view for a panorama?

Firstly, if the scene is a 360 deg spherical panorama, don't select Partial, select Full 360. If you want to make only 180º for the 360 deg scene, go to the scene and adjust the pan values on the properties panel of the scene, that is the min. and the max. For example, adjust the min. to -30, max. to 150, then you'll get a 180 deg panorama.
Secondly, if the panorama itself is a partial panorama, select "Partial" for the Panfov in the properties panel of the scene first and then adjust the pan values.