Welcome to Android Central! If you're talking about trying to install apps from the Play Store, then try this:
Try going to Settings>Apps, select Google Play Store, tap Storage, and Clear Cache/Clear Data, then Force Stop. Do the same for Download Manager -- you probably have to tap Menu>Show System to see it. 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.