What did you do to make it not download or install as it is more likely than not is user induced problem? Did you download/install it from different source? How many profiles you have? Permissions? Did you try removing it first?
It's the same as posting: why cant I breath whiteout providing any details - are you under water? Is your mouth open? are you alive?
My point is - please provide pertaining details in order to solicit helpful replies from knowledgeable and willing to help people instead of BS replies from assholes like me