I am trying to draw a circle with coordinate plane using loops in C programming but ıt does not give me the lower part of circle

You program should be more like this:

#include <stdio.h>
#include <math.h>

int main() {
    int i, j, k;
    int a = 3, b = 4, r = 5;
    int x, y;
    
        for (i = 0; i < 21; i++) {
            y = -(i - 10);

            if (y == 0) {
                for (j = 0; j < 21; j++) {
                    x = j - 10;

                    if (r * r == pow(x - a, 2) + pow(y - b, 2)) {
                        printf("*");
                    }
                    else if (x == 0) {
                        printf("|");
                    }
                    else {
                        printf("-");
                    }
                }
            }
            else {
                for (k = 0; k < 21; k++) {
                    x = k - 10;

                    if (r * r == pow(x - a, 2) + pow(y - b, 2)) {
                        printf("*");
                    }
                    else if (x == 0) {
                        printf("|");
                    }
                    else {
                        printf(" ");
                    }
                }
            }
            printf("\n");
        }

}

When you match your equation, you should allow for (-3) ^ 2 is 9, while if you take the square root of 9, you could only match a 3, unless if you also check whether the "minus" of the square root is a 3, which is troublesome.

It might also be good that if you use x and y and do the calculations, and then at the last moment, map to the screen's method of displaying it, or a similar way, instead of using i and then -(i - 10) or 10 - i every where to mean y, so I added the x and y in your program. You might also comment that (a, b) is supposed to be the center of the circle.

There is also one point about, it seems the difference between y being 0 or not is that you print out the hyphen (for the x-axis), vs a space, so your program could be:

#include <stdio.h>
#include <math.h>

int main() {
    int i, j;
    int a = 3, b = 4, r = 5;
    int x, y;

    for (i = 0; i < 21; i++) {
        y = -(i - 10);

        for (j = 0; j < 21; j++) {
            x = j - 10;

            if (r * r == pow(x - a, 2) + pow(y - b, 2)) {
                printf("*");
            }
            else if (x == 0) {
                printf("|");
            }
            else {
                printf(y == 0 ? "-" : " ");
            }
        }
        printf("\n");    
    }
}

and in fact, you don't need i and j, but can directly iterate through x and y, as your way of displaying is only one character each time:

#include <stdio.h>
#include <math.h>

int main() {
    int a = 3, b = 4, r = 5;
    int x, y;

    for (y = 10; y >= -10; y--) {

        for (x = -10; x <= 10; x++) {
            if (r * r == pow(x - a, 2) + pow(y - b, 2)) {
                printf("*");
            }
            else if (x == 0) {
                printf("|");
            }
            else {
                printf(y == 0 ? "-" : " ");
            }
        }
        printf("\n");    
        
    }
}