You can only restore a backup onto a version of iOS that is at least equal to the one that was used to create a 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.