Declareing array that stores a struct, which defined in another file included, gives the error "Incomplete Type "Array[]" is not allowed"

I have defined a struct in another file called file1.h (example) and when I declare an array using that struct in another file called File2.h, i get the error Message:

"Incomplete Type "array[]" is not allowed"

Example Code:

File1.h:

#ifndef MYSTRUCT_H
#define MYSTRUCT_H
    
struct MyStruct 
{
    float number1;
    float number2;
};
#endif

File2.h:

#ifndef MY_CLASS_H
#define MY_CLASS_H
#include "File1.h"

class MyClass 
{
public:
    MyStruct array[]; // I get the error here
};

#endif

In C++, a member of a struct or class which is of an array type must be declared with a compile-time size.
(this is different in C which has a flexible array member - see here).

Therefore this will compile:

#include <cstddef>  // for size_t

// ...

class MyClass {
public:
    static constexpr size_t sz = 10; // compile time constant
    MyStruct arr[sz];
};

Live demo 1

However:
In C++ it is usually recommended to use std::vector for dynamic size arrays, and std::array for static size ones.

You seem to require the array to be of dynamic size, and therefore you should use std::vector:

#include <vector>

// ...

class MyClass {
public:
    std::vector<MyStruct> arr;
};

Here arr can be dynamically resized whenever you need.

Live demo 2

Note:
Your array is not a very good name for a member. I renamed array to arr to avoid collision with std::array.