Must be a vector

To Mathcad a vector is a column of numbers, not a row!

You try to access "min" with just one single matrix index (i-1 in your example). This is only allowed for (column) vectors (= n x 1 matrices) because here it is clear that the index means the row as there is only one column.

In your sheet "min" is a 24 x 9 matrix, so you need two indices (row and column) to access one of its elements.

As you are only using one index, Primes error message tells you that this is only allowed if min is a vector (= a matrix with just one column).