This error is appearing because you have an integration setup, because of certain limitations with integrating the two systems you will need to cancel the sustaining gift and record a new one with the proper duration. Unfortunately this is a necessary limitation for both the LCRM and Raiser's Edge Integrations. However, if you have the Raiser's Edge integration the ability to change the duration of a sustaining gift may be added in the future -- this is being tracked in bug 68055. If you would like to be notified if/when this feature is added, please file a case with support to be added to the bug.