JsonSerializer.Deserialize always returns Null

I think you need to familiarize yourself more with the Json structure, Material is not the type of object being returned in your Json string, it is a nested class of the main object, I have put together a dot net fiddle and tested that this will work for you, as you can see, the main object is what I called MyJson which holds one property of class type MetaData and a property name of d, this class then contains 3 properties: of type MetaBase which contains 3 properties of string types and 2 other string properties, I then deserialize the json string to the main object type of MyJson and then write the nested property MATNR to the console window, this prints 250, I’m also using, JsonPropertyNameAttribute to set certain class properties to the matching values in the json string, you can read more about that here:system.text.json.serialization.jsonpropertynameattribute
fiddle here: dotnet-fiddle:

using System;
using System.Text.Json;
using System.Text.Json.Serialization;
public class Program
{
    public static void Main()
    {
        string json = "{"d":{"__metadata":{"id":"http://my.dev.int:8000/sap/opu/odata/sap/ZFIORI_SERIAL_NUMBERS_SRV/MATERIALSet('250')","uri":"http://my.dev.int:8000/sap/opu/odata/sap/ZFIORI_SERIAL_NUMBERS_SRV/MATERIALSet('250')","type":"ZFIORI_SERIAL_NUMBERS_SRV.MATERIAL"},"MATNR":"250","MAKTX":"X:K10/MF250"}}";
        var myjson = System.Text.Json.JsonSerializer.Deserialize<MyJson>(json);
        Console.WriteLine(myjson.D.MATNR);
    }
}

public class MyJson
{
    [JsonPropertyName("d")]
    public MetaData D {get; set;}
}

public class MetaData
{
   [JsonPropertyName("__metadata")]
   public MetaBase MetaB {get; set;}
   public string MATNR { get; set; }
   public string MAKTX { get; set; }    
    
}

public class MetaBase
{
   [JsonPropertyName("id")]
   public string ID {get; set;}
   [JsonPropertyName("uri")]
   public string Uri {get; set;}
   [JsonPropertyName("type")]
   public string Type {get; set;}
    
}