Assignment to expression with array type error

1. You need to compile your code with warnings enabled, the will guide you through debugging step by step. Try to eliminate one, and compile again. In GCC for example, you would use -Wall flag.
2. The numerical fields of your struct should be just numbers, not arrays. Moreover, having their type as long long int seems a bit too much, but I leave that on you.
3. Having (ptr + y)->Savings to access the y-th element of an array of structs and the its field names Savings is technically correct, but it's much more cleaner (thus increases readability and maintainability) to write ptr[y].Savings. That is a general rule, applying to the rest of your code.
4. I believe the above led you to make two syntactical errors when you were reading the numerical data of the customers with scanf(), since you know that an integer in general needs the & operator. If you had used the clean approach from the start, you wouldn't made those, I believe.
5. For long long int use the %lld format specifier, not just %d.
6. In Bubblesort, when you find elements that need to be swapped, then swap the whole elements, not just their Savingss. I recommend creating a function to do that.
7. scanf("%[^\n]s" doesn't make much sense, I would change it to scanf("%99s", where 99 is the maximum size of your string, minus one. Read more in the 2nd paragraph in scanf(“%[^\n]s”,a) question.

Putting everything together, we get:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct Data {
    long long int Savings, AccNo; // these should be numbers, not arrays
    char Name[100];
};

void swap(struct Data* a, struct Data* b) {
    struct Data tmp;
    tmp.Savings = a->Savings;
    tmp.AccNo = a->AccNo;
    strcpy(tmp.Name, a->Name);

    a->Savings = b->Savings;
    a->AccNo = b->AccNo;
    strcpy(a->Name, b->Name);

    b->Savings = tmp.Savings;
    b->AccNo = tmp.AccNo;
    strcpy(b->Name, tmp.Name);
}

int main() {
    struct Data *ptr;
    int n, i, x, y;

    printf("Enter number of customer(s) : ");
    scanf("%d", &n);

    ptr = malloc (n * sizeof(struct Data)); // do not cast malloc

    for(i=0; i<n; i++) {
        printf("Enter customer %d", i+1);
        printf("\nName : ");
        getchar();
        scanf("%99s", ptr[i].Name); // ptr is a pointer, but now you want to actually use it as an array, so use '.'
        printf("Savings : ");
        scanf("%lld", &ptr[i].Savings); // needs &, just like you did for 'n'. USe `%lld' for long lont int as the format specifier.
        printf("Account Number : ");
        scanf("%lld", &ptr[i].AccNo);   // needs &, just like you did for 'n'. USe `%lld' for long lont int as the format specifier.
        printf("\n");
    }

    //Sorting bubblesort
    for (x = 0; x < n; x++)
    {
        for (y = 0; y < n - x - 1; y++) // you don't need paranetheses in the stop condition
        {
            if (ptr[y].Savings > ptr[y + 1].Savings)
            {
                swap(&ptr[y], &ptr[y + 1]); // swap the whole element, not just its savings
            }
        }
    }

    //Print sorted
    printf("\nSorted customers are:\n");
    for(i=0; i<n; ++i)
    {
        printf("%s\n", (ptr+i)->Name);
        printf("%lld\n", (ptr+i)->Savings);
    }
    free(ptr);
    return 0;
}

Output (with relevant input):

Sorted customers are:
George
1
Babis
3
Theodor
20

PS: Do I cast the result of malloc? No!