Indexing into a Matrix, Array, or Vector must be done with integer values. Related to this, multi-element selection with a Matrix, Array, or Vector must also come down to integers at the endpoints. Trying to index a Matrix with a symbol will result in an error.

Example 1

Note that $n$ has no value in this example.

Solution:

In this case, defining $n$corrects the error.

Example 2

This one is more subtle because the sum command is following Maple's normal evaluation rules. The index operation $V\left[i\right]$ is attempted before sum is called.  At this point, the name $i$ does not have a value.

Solution 1:

Evaluation of $V\left[i\right]$needs to be delayed using right single quotes.   

Solution 2:

Instead of sum, use add.  The add command is recommended for adding a finite sequence of values.  The add command has special evaluation rules, so no error occurs in the evaluation.

Example 3
A nested call to sum with only one set of right single quotes leads to a different error. A second set of right single quotes corrects the problem.

Solution: