What does "I am having a trouble with installation" mean? Please provide details so that we may be able to assist you.
It means, I follow steps to download Adobe Flash, but it won't install. And one of the pop-up windows on Safari "told" me to check Security settings (?)
If there weren't any errors during installation then it's a good sign the installation was successful. Apple disables plugins by default in Safari 10, so what you saw was probably a notification to this effect. Please see Enable Adobe Flash Player for Safari for more information.