Visual Studio Code Include path error in C - c

Question

It seems to be a simple problem but I can't find a solution to it.
I'm having a issues while including the header of a basic function I created.
main.c:2:10: fatal error: 'add.h' file not found
#include "add.h"
I made this code as simple a possible to highlight the main issue.
Here's my main code:
#include<stdio.h>
#include "add.h" //add.c function header
int main(int argc, char const *argv[])
{
int num1 = 5, num2 = 6;
add(num1, num2);
return 0;
}
add.c function code:
#include <stdio.h>
void add(int n1, int n2)
{
int sum = n1 + n2;
printf("%d + %d = %d\n", n1, n2, sum);
}
And the add.h
void add(int n1, int n2);
My c_cpp_properties.json file.
{
"configurations": [
{
"name": "Mac",
"includePath": [
"${workspaceFolder}/**",
"${workspaceFolder}/header"
],
"defines": [],
"macFrameworkPath": [
"/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/Frameworks"
],
"compilerPath": "/usr/bin/clang",
"cStandard": "c11",
"cppStandard": "c++17",
"intelliSenseMode": "clang-x64"
}
],
"version": 4
}
Project structuration:
Test_Header(folder)
source(folder)
main.c
add.c
header(folder)
add.h
I'm on Mac, my editor is Visual studio Code and I run the program with the run button provided by the "code runner" visual studio code extension.
I have a little light bubble on the top of my #include "add.h" saying "add to include path".
My guess is that it comes from the compiler, like he's not looking at the right place for the headers.
How can I change that (if that's what I need to do)?
Also, do changing it wouldn't change where he's supposed to find the built in c function library , like stdio ?
Or he can understand that he has a folder for the standard c function library and an other for user custom headers that should be located in the project directory ?
I've been 3 days on that issue, Couldn't find an answer on the web, or stuff that I couldn't understand. I could switch on an IDE but it will be like giving up.
I'm fairly new in the world of programmation as you can guess, so if you can keep it as simple as possible, it would be great :).
Thank you.