The sum command (sum) is for symbolic summation.

You can enter the command sum using either the 1-D or 2-D calling sequence.  For example, sum(k^2,k) is equivalent to $\sum _k{k}^2$.

Computes a closed form for a sum.

Maple returns the call unevaluated if it cannot find a closed form.

To add a finite sequence of values, rather than compute a formula, use the add command.  For example, add(k, k=0..9) returns 45.  Although the sum command can often be used to compute explicit sums, it is strongly recommended that the add command be used in programs if an explicit sum is needed, in particular, when summing over all elements of a list, Array, Matrix, or similar data structure.