Set tempo from event AFAIK sets it from the definition in the event (sample editor → definition). It has to be a track with defined tempo.
If you want to set the tempo on a short track, for example a loop, you should cut the sample and bounce it so it fits a certain set of bars/beats and then in the Sample Editor, set the bars/beats in the top bar.
If it is a long track, what you should do is Project → Tempo Detection which will set the tempo track based on the tempo Cubase analyzes from the event. It lets you use time warp (not audio warp!) to place the beats on the correct spots, to fix tempo variations. The tempo detection works well on beats and full mixes, but not as good on other instruments. It is meant to be used with long pieces of audio.
Afterwards, if you want to, you can use “Set definition from tempo” to set the definition of the event from this calculated tempo. This will place the event in musical mode and let you then modify the tempo track to you liking, which will cause audio warping on the event.