I faced the same scenario once, Where as the index is not available in search head.(I am not sure why/how this happen).
you will came to know the same by any of the below options,
Option 1: Go to search head Settings-> Data -> Indexes and check your index is available or not.
1. create a search
2. schedule it based on your requirement
3. check the summary indexing check box
4. you can see the list of indexes available for summary. I guess the index which you are referring will not be available.
In that case you need create a same index in search head that will work.