I am aggregating and then restructuring my data set in preparation for
a repeated measures anova. I've done this before on another data set
without difficulty. Now, I find that I get the error message: "An
invalid index value was detected at case 1. String index values may
not be blank. Numeric index values must be non-negative integers." So it won't do the restructure.

I have looked at my data and I have no string data, and all of the
numbers in my numeric data are non-negative numbers.

What am I doing wrong? Any help appreciated!!!!

Have you tried running summary statistics on the data table? Sometimes that will quickly identify numbers that are being read as strings or similar type issues.