could somebody tell me how I fix an index error?

[buran]

The problem is not with the loop, but the MonthlyReport.txt file you create. You write to this file in 3 locations and you never use ; e.g. as separator. Unless there is ; in your data, which I doubt, it will throw error because split will return 1-element tuple. i.e. there is only index 0. And even if there is ; in your data, I doubt that is what you want.
Probably you want to use ; as separator, not \n.