Hi,
Additional to Sontosh comment check if your previous condition record has been deleted.
that means when you have putted the new condition record it may happen you have deleted the previous one and then added the new condition record, So in that case the change sales order do not get any condition record . so it giving you error.
better close it by validity and keep it in condition record. else previous orders will have problem when you want to change some thing,.
Regards
Pitabash