1 item could not be synced. See iTunes for more information
iTunes displays an alert when it is unable to sync specific audio or video content to your iPhone, iPad, or iPod. iTunes offers a checkbox to not show the alert again. If you select this checkbox, the alert will no longer appear during sync.

Connect your device to your PC or Mac.
Wait until your device has appeared on the left side of the iTunes window under Devices.
Right-click (PC) or Control-click (Mac) the icon for your device.
From the shortcut menu, choose Reset Warnings.