Hello and welcome to justanswer. My name is***** will be assisting you today. I am sorry to hear that you are having such trouble.
conflicting application is blocking this. Run the Activity Monitor to stop it.
Go to Applications > Utilities to find Activity Monitor, and launch it by double-clicking its icon. (The Applications folder is on the root level of your hard drive.)
In the Activity Monitor, select the process with the full name SafariNotificationAgent, then click the upper-left button to close the process.
Pleas try the download again and let me know,