You need to exit your while loop when sum_val > 999. If you look at your exit condition and read it as words, you can see your problem:

while sum_val > 999 is "Loop and perform loop statements while the value of sum is greater than 999".

Since your sum starts at 0, you will never enter your loop, because sum is less than 999. You want:

"Loop and perform loop statements while the value of sum is less than 999"

which is:

while sum_val < 999

As well as this you don't define your counter n. You do this:

sum_val=1+n

When you count your sum, you need to ensure that you are actually summing the right things. What is sum it is the previous value of sum added with the current value of n. So do that:

sum_val = sum_val + n

Also what is n? Well as in the problem statement, n counts from 1 to N (your eventual goal). So you need to make sure you update that (and set it).

n = 1
...
n = n+1

Finally, sum is a function name, so you cant use it. Use something else, like sum_val as shown above.

You will need to sort out where in the code these should go.