It's not that which I was drawing attention to. I am expecting OP to get a compiler error message:

Yes, I also saw this, but this is one of many error in this code extract:

• index out of bounds
• bad types
• wrong assignment statement
• ...

Impressive how many errors can be done in less than 100 lines of code :D

And in your code you are trying update item 31 of name and you are surprise that this do not work!!

It's not that which I was drawing attention to. I am expecting OP to get a compiler error message:

struct student{
    char name[30];
}
record[0].name[30] ="mangal";

The type of name[30] versus the type of "...".

But i not know why ?

Are you understanding what you are coding?
You have defined a struct which constains an int value called id, a float value call percentage and an array of char called name which have 30 elements.

And in your code you are trying update item 31 of name and you are surprise that this do not work!!

Are you serious?

@JonB error is telling me assignment is not possible. But i not know why ?

For goodness sake, that is not an error message.... Really how do you expect people to help you?

You just can't do "record[0].name[30] ="mangal";"
As the contents are modifiable, the arrays themselves are not.
Use strcpy from <string.h> instead.

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

struct student{
    int id;
    char name[30];
    float percentage;
};

int main(){
    
    int i;
    
    struct student record[2];
    
    // 1st student's record
    record[0].id=1;
    strcpy(record[0].name,"mangal");
    
    // 2st student's record
    record[1].id=2;
    strcpy(record[1].name,"himanshu");
    
    //test
    std::cout << record[0].name << std::endl;
    std::cout << record[1].name << std::endl;
}

Just one hint - I would do the "record = id" ;-) its easier to handle.

when i use it in below code it throw error. i want to know technical reason behind that .

Seriously? Do you think posting the error message you are receiving from the compiler would be relevant, or should we guess?

record[0].name[30] ="mangal";

The most basic C/C++. What type is record[0].name[30]? I'm quite sure whatever error message you are receiving tells you this....