dotnet 6 unable to find async

the title said it all notnet6 is unable to find async, this is the error I get

/home/user1/Documents/GitHub/YTD/Core/main.cs(19,16): error CS0246: The type or namespace name 'async' could not be found (are you missing a using directive or an assembly reference?) [/home/user1/Documents/GitHub/YTD/Core/Core.csproj]

code

public async Ytd(string[] args)
    {
        Console.WriteLine(0);
        response = "https://www.youtube.com/playlist?list=PLda3VoSoc_TSBBOBYwcmlamF1UrjVtccZ";//args[0];
        extention = args[1];
        await Download();

    }

GitHub repo: https://github.com/Kaifungamedev/YTD

>Solution :

You need to specify the return type of the function.

public async Task Ytd(string[] args)