Welcome to the forums! Somehow, you've ended up on the Community Canvas, which is a sub-forum for discussions related to the online community and the forums platform. No worries! But you'll want to post further questions seeking Tableau help here: Forums
As far as why the legend isn't visible... it's a good question. Are you looking for a color legend? In that case, you'll need to have a field on the Color shelf. A size legend? Then a field on Size. A shape legend? Then a field on shape.
Most of the time the legends will show when you drop the field on the appropriate space in the view. But sometimes legends get hidden. In that case you can right click a gray area in the view and find Legends. Or from the menu you can select "Analysis" --> "Legends" and then show the desired legend.
On a dashboard, if a legend isn't showing for a particular view, you can look for the little drop down arrow/caret in the upper right corner of the view on the dashboard and then select the legend you want to show from there.
Hope that helps!