The input is not a valid Base-64 string

It's not a Base64 encoded string. It's a Guid. You can read it into a byte array like this

var bytearray = new Guid("59216167-f9c0-4b1b-b1db-1babd1209f10").ToByteArray();

The input is not a valid Base-64 string

The exact reason you are getting this type of error is because it's not a valid Base64 string, instead as already mentioned, it's a Guid; and not a valid Guid at that.

First you can check if you even have a valid Base64 string by trying to convert it.

public static bool StringIsBase64(string myString)
{
   Span<byte> buffer = new Span<byte>(new byte[myString.Length]);
   return Convert.TryFromBase64String(myString, buffer , out int bytesParsed);
}

Now if you call this function and it succeeds, then we would assume you do have a valid Base64 string, otherwise a conversion error would occur.

Your call can now look like this:

 string id = "59216167-f9c0-4b1b-b1db-1babd1209f10@ABC";
 var bytes;
 if (StringIsBase64(id))
 {
    bytes = Convert.FromBase64String(id);
 } 

Something else I would like to address that none of the other answers addressed, is the input is not valid for even a Guid. A GUID is a 128-bit integer (16 bytes) and that string isn't valid.

You actually would receive the error:

Guid should contain 32 digits with 4 dashes (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx)

The characters @ABC at the end of the string are causing this, if these are removed then we have an actual valid Guid.

Try this will Help

    using System;
    public class Program{

    public static void Main()
    {
        Guid gg = Guid.NewGuid();
        Console.WriteLine(gg);
        string ss = Encode(gg);
        Console.WriteLine(ss);
        Console.WriteLine(Decode(ss));
    }
    public static string Encode(Guid guid)
    {
        string encoded = Convert.ToBase64String(guid.ToByteArray());
        encoded = encoded.Replace("/", "_").Replace("+", "-");
        return encoded.Substring(0, 22);
    }

    public static Guid Decode(string value)
    {
        value = value.Replace("_", "/").Replace("-", "+");
        byte[] buffer = Convert.FromBase64String(value + "==");
        return new Guid(buffer);
    }




}