Why can't I download/install Adobe Flash Player?

About my mac

Macbook Air (Early 2014)

I'm on the version 10.9.3 (Mavericks)

1.4 GHz Intel Core i5

I just had gotten my macbook air about 4 days ago. So when I wanted to watch yourube videos, it won't let me and it says I need to download adobe flash player and so I did. I followed the steps and all. It downloaded till 97% and stopped. It said that I need to close the following programs which is safari and so I did. But then I retry to do it again it won't budge. I kept spamming the retry button but it still didn't proceed to finish the download. So I thought that I need close Safari since the start and I did that. But as it reached 97% it stopped and said I needed to close Safari I didn't even open Safari! I kept uninstalling the adobe flash player then installing it again but nothing work. And I think it's making my mac kinda laggy but I don't know if the process of installing and uninstalling it is making my mac laggy.

But then yesterday, I was searhing on Google for a BATG S3 Episodes and it lead me to YT so I just wanted to try my luck and maybe a miracle could happen and I could watch it. And then as I click on the video, it played! I was so thrilled but after a couple of videos it went back to "You need to download abobe flash player" .I thought it was kinda weird since I uninstalled adobe because I was getting ******.

Help me please. Thank you.