Whenever I am trying to open an app, most of them won't open with the error as below.
I am really helpless here. Is there is any issue with my machine?
I have a Ubuntu background and recently started using OS X.
Mac version :
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Apple let a vital App Store certificate apply, unfortunately. Update your OS and that should fix the issue.
While this does not speak well for Apple, many OS manufacturers have had critical (or if you will stupid) flaws in their OS. Which makes it a good idea to keep up to date on released updates in the App store.