This error means that some line(s) in your flowfile have less than 4 fields. Empty lines are typically ignored but if you have a trailing line, for example, with a space in it, that space will be interpreted as the first field (GradeNm) and the other fields will be missing, which could lead to this sort of problem.
Check your input file for trailing lines or lines with less than 3 commas in it.
If the problem is being caused by a blank line with spaces, for example, you could use the ReplaceText processor to remove those spaces, as shown below:
Cheers,
André
