dftr Data Format

You can't use Sample[i (which is just a single scalar) in the "dftr" function (the argument must be a vector or matrix).

You have to use the "submatrix" command!

EDIT: Ah, ttokoro was faster 😉

Here is a suggestion for doing it:

The results are

Note that the range i must run up to rows(Sample)-12 and not up to rows(Sample)-1 as in your attempt.