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Have you tried restarting your Mac ?
First turn off your Mac and turn it back on (by holding down the Power button for 10 seconds). This should fix the problem. If that didn't work, then do the steps below
In Finder, click on the Finder icon, click on Go menu at the top bar and select Go To Folder. Type the following and press return
This will open Library. Open Updates folder. Open all folders you see in there and if you see any files similar to safari (like Safari.pkg), open it and install it. If that didn't work, then go to Applications > Utilities folder and open Activity Monitor. If you see "storedownloader", select it and quit it. Restart your Mac.
Restart your Mac. Please let me know if that didn't work, so we can continue troubleshooting.