Welcome to Android Central! Is this happening on Google Play Store, the Galaxy Apps Store, or some other app store? Is this on both wi-fi and mobile data?
If it's Google Play Store, then try going to the App Manager, select Google Play Store, and Clear Cache/Clear Data, then Force Stop. Do the same for Download Manager. Now try again.
If that doesn't work, try going to Settings>Accounts-Google, select your Google account, and remove it completely from the phone. Then add it back again, and see if that helps.