What is the problem of these (if else function) and how to improve it?

I see two issues with your conditional expressions. The indentation is a bit messed up so that makes the second one a little harder to see. Here is your logic with revised indentation :

#include <stdio.h>
#include <stdlib.h>
#include<string.h>
#pragma warning(disable:4996)

int main(void)
{
    // Variable declarations
    int t = 0;
    float k, y, u, i;
    int a, b, c, d;
    int p;
    float  add, tt, f;
    char name[30], ic[20];
    char addr[50];

    //The personal details of customer //
    printf("\n=================================================================================\n");
    printf("\n Vegetable ordering system\n");
    printf("\n=================================================================================\n");
    printf("Please enter your full name     :");
    scanf(" %[^\n]", &name);
    printf("Please enter your ic number     :");
    scanf("%s", &ic);
    printf("Please enter your phone number     :");
    scanf("%d", &p);
    printf("Please enter your home address     :");
    scanf(" %[^\n]", &addr);
    printf("\n==================================================================================\n");
    printf("The customer name is  %s  and the ic number is %s \n", name, ic);
    printf("The customer who is come from come from %s", addr);
    printf("\n==================================================================================\n");

    //Order part of program//
    printf("\n==================================================================================\n");
    printf(" Enter the order quetity purchase \n");
    printf("\n==================================================================================\n");
    printf("Please order the package A        :");
    scanf("%d", &a);
    printf("Please order the package B        :");
    scanf("%d", &b);
    printf("Please order the package C        :");
    scanf("%d", &c);
    printf("Please order the package D        :");
    scanf("%d", &d);
    printf("\n==================================================================================\n");

    if (a, b, c, d > t)
    {
        printf("%d", a - t);

        k = (24.5) * a;
        printf("\nPackages  A              :RM%.2f\n", k);

        y = (26.5) * b;
        printf("\nPackages  B              :RM%.2f\n", y);

        u = (26.5) * c;
        printf("\nPackages  C              :RM%.2f\n", u);

        i = (24.5) * d;
        printf("\nPackages  D             :RM%.2f\n\n", i);

        //The addition part for total cost for customer to pay//
        add = k + y + u + i;
        tt = add + 5.90;
        f = tt - (0.15 * tt);

        if (add > 100)
        {
            printf("Total                                              =RM%.2f\n", add);
            printf("The delivery fee which is fixed                    =RM5.90\n          ");
            printf("\n==================================================================================\n");
            printf("The total price for customer need to pay with 15% discount          =RM%.2f\n", f);
            printf("\n==================================================================================\n");
        }
        else (add<100)
        {
            printf("Total                                              =RM%.2f\n", add);
            printf("\n==================================================================================\n");
            printf("The total price for customer need to pay       =RM%.2f\n", tt);
            printf("\n==================================================================================\n");
        }
        system("pause");
        return 0;
    }
}

I revised the braces on the second if and the else that check add. That may not be correct but your intentions were difficult to decipher with the code as it was.

The first else expression is unlikely to be what you intend. Here is a guess at your actual intent :

if( ( a > t ) && ( b > t ) && ( c > t ) && ( d > t ) )
{
}

or you might want to test each one independently :

if( a > t )
{
    k = (24.5) * a;
    printf("\nPackages  A              :RM%.2f\n", k);
}
if( b > t )
{
    y = (26.5) * b;
    printf("\nPackages  B              :RM%.2f\n", y);
}
if( c > t )
{
    u = (26.5) * c;
    printf("\nPackages  C              :RM%.2f\n", u);
}
if( d > t )
{
    i = (24.5) * d;
    printf("\nPackages  D             :RM%.2f\n\n", i);
}

I find it helpful to use braces almost all the time, even when it is not necessary. It can make your code a little easier to read and it is definitely easier to modify and maintain.