Why C# Error CS0103 when calling a method from another namespace when the namespace was rightly specified?

I created an age conditional "Hello World!". You type in your name, then I ask your age and if you're under 8, I ask you to leave the computer because you should just go play outside.

Here's the file:

using System;
using System.Security.Cryptography; //honestly don't know why this is here
using FileManagement;   // I'm going to talk about this later

namespace P2C2._1;

public class ConditionalHello
{
    public static void SayHello(string receiver)
    {
        Console.WriteLine("Hello " + receiver + " !");
    }

    public static void AgeCondition(string receiver)
    {
        Console.WriteLine("How old are you?");
        int age = Convert.ToInt32(Console.ReadLine());

        if (age <= 8)
        {
            Console.WriteLine("A nice day awaits outside the computer :)");
        }
        if (age > 8)
        {
            SayHello(receiver);
        }
    }

    public static void Main(string[] args)
    {
        // main function, say hello if you are old enough
        Console.Write("Enter the receiver's name : ");
        string receiver = Console.ReadLine();
        AgeCondition(receiver);

        // (temp lines)
        // appending the README from OpenClassRoom to explain the transformation of P2C2.1
        // 1. content creation
        string content = "# Originally an OpenClassRoom file, from their course 'debuting w\ C#" +
            "***" +
            "I transformed it to add an age condition to cast away the children under 8" +
            "Next, I need to add a function for when the age entered isn't a number.";

        // 2. appending the file
        string fileName = "README.md";
        WriteFile(content, fileName); //error CS0103 right here
    }
}

Originally, this was a code from an OpenClassRoom course. That's why I don't know the using System.Security.Cryptography and also why I want to append the original README. I meant to use the function WriteFile just for once.

I notice the code was too busy so I created a new file for the WriteFile method, FileManagement.cs, and here's its script:

using System;

// adding text to the og README file

namespace FileManagement;

public class FileManagement
{
    public static void WriteFile(string content, string fileName)
    {
        try
        {
            string filePath = @"C:\Users\(lots of folders)\P2C2.1\README.md";
            File.AppendAllText(filePath, Environment.NewLine + content);
            Console.WriteLine("Writing done in the file" + fileName);

            string fullPath = Path.GetFullPath(fileName);
            Console.WriteLine($"You can find the file in: {fullPath}");

        }
        catch (UnauthorizedAccessException e) //there might be too much catch here
        {
            Console.WriteLine("Non-authorized access: " + e.Message);
        }
        catch (IOException e)
        {
            Console.WriteLine("E/S error: " + e.Message);
        }
        catch (Exception e)
        {
            Console.WriteLine($"Error while writing in the file: {e.Message}");
        } 
    }
}

Why can't I just initiate WriteFile in my Main function when I put on "static" before the WriteFile method?

I tried to:

• put static to the class;
• put brackets to actually form the namespace in the FileManagement.cs;
• copy/paste the namespace FileManagement and method WriteFile to each occurence to prevent tipo ;
• read the Microsoft documentation here that said it could happened if the method is defined in try/catch but I think I cannot do otherwise in my case;
• read a real similar issue here on Stack Overflow, C# error CS0103 when using methods in different classes, but very much complex code so I didn't understand anything.