What type of accounts do you have connected to your phone? Outlook? Google? etc. Look at "Settings/Accounts/Email & App Accounts" and you'll see the list of connected accounts for Email/Calendar/Contacts.Then go back to "Settings/Accounts/Sync Your Settings" and make sure that "Sync Settings" is 'On'.
Hi April,
1) firstly you must add your Microsoft account
2) even after first step, my lumia kept giving me same problem exactly as yours till windows & People App get updated