I'll keep it simple.

Documentation really does NOT matter here. The intent seems pretty clear.

Sum all integers while using a VLA.

Header is fine. The most complicated thing happening here is I/O.

Return can be cast, it's personal preference.

You can use return 0; or return (0); or return(0);

and if you're really feeling saucy; as horrific as it may seem and startle the community into mass pandemic of terror and anxiety... you can include stdlib.h, void main(void), and use exit(0).

Even if short is 2 bytes, you still have up to a range of -65,537 to 65,536 possible values. Assuming most systems can handle a 4 byte int, long long isn't really necessary. unsigned, in this context, can cause serious issues in even the most basic of concepts such as this one. You should allow for signed values. I'll explain why in a bit. But for now, we'll change this

unsigned short int num = 0;

to this

int num = 0;

The while() loop is good. If num is <= 0 or num is >= 256 prompt the user again. Your filtering the input, but there is a catch with scanf() and its that it reads up until the first whitespace character. You need a way to clear the input.

while (getchar() != '\n'); or while (getchar() != '\n') continue;

usually solves this problem. That way, any invalid input is cleared and new input is ready. Also, you note that while num is less than zero. If your num variable is of type unsigned, you've created a semantic error. One that took me awhile to get used to too. It's incredibly easy to make it as well. That's why it should be signed and not unsigned. unsigned types have their place and this context is not it.

while(num < 0 || num > 255)
{
    printf("Enter number of values to sum: ");
    scanf("%d", &num);
    while (getchar() != '\n') continue; //clear the input buffer
}

as for the vla arr, the total variable, and the i variable...

unsigned long long int arr[num];
unsigned long long int total = 0;
unsigned short int i;

should really be

int arr[num], total, i;

you don't even really need to declare i, but if you want compatibility, you can declare i outside of the for() loop. Again, it depends on context and preference.

again, we don't need a long long type. just an int type.

    scanf("%lld", arr + i);

should really just be

scanf("%d", &arr[i]);

since we are working with arrays, not pointers (even though, an array is a type of pointer), the intent is clearer and consistency runs throughout your code; and remember, were storing the value into that array member.

we don't need to offset num by 1 since i needs to be less than num. The elements are already offset by one and initializing i to 0 accomplishes that goal. but really, we don't even really need that code at all. all we need to do is print the total.

for(i = 0; i < num; i++)
{
    scanf("%d", &arr);
    while (getchar() != '\n') continue; //clear the buffer
    total += arr[i];
    printf(" + ");
}
printf("total sum is %d\n", total);

but you should get the idea by now.

in all, this is my revised code. while, i'll admit, it is very buggy and unstable. its a basic framework to play with and displays my point of clearing whitespace produced by using scanf(). it accomplishes the task of creating a vla and the data types used fall well within the bounds created by the initial input.

assuming vlas are being used, we can assume that a c99 compliant compiler is being used.

#include <stdio.h>

int main(void)
{
    int num = 0;

    printf("Enter number of values to sum: ");
    while(1 != scanf("%d", &num) && (num < 0 || num > 256))
    {
        printf("Enter number of values to sum: ");
        while (getchar() != '\n'); //clear buffer
        scanf("%d", &num);
    }
    while (getchar() != '\n');
    
    int arr[num], total = 0;
    
    printf("%d number(s) to be summed: ", num);
    for(int i = 0, j = num - 1; i < num; i++, j--)
    {
        while (1 != scanf("%d", &arr[i]))
        {
            printf("%d number(s) to be summed: ", j);
            while (getchar() != '\n') continue; //clear buffer
            scanf("%d", &num);
        }
        while (getchar() != '\n');
        total += arr[i];
        if (j != 0) printf("%d number(s) to be summed: ", j);
    }
    printf("The total sum is %d\n", total);

    return 0;
}