You can only restore a backup onto a version of iOS that is at least equal to the one that was used to create that backup. For instance if your old iPhone was running iOS 10.3.3, then you can only restore its backup onto another iPhone also running iOS 10.3.3, or iOS 11.
Replacement iPhones & iPads are often running a version of iOS that is several months old. You will need to update it to iOS 10.3.3 before iTunes will allow you to restore it from backup.