The error is extremely clear but your design is very confusing and incomplete. The nullable sbyte type (signed byte) has 256 different values and null while your design is looking for a 0 or 1 and does not check for null.
What happens if the value is null or a value other than 1 or 0? Perhaps an Enum is a better design approach?
Enumeration types (C# reference)
Example code. Rather than a static method you could take advantage of an extension method. Rather than an if you could use a case.
public static async Task Main()
{
sbyte? status = null;
string result = GetStatus(status);
Console.WriteLine($"Result: {result}");
status = 1; result = GetStatus(status);
Console.WriteLine($"Result: {result}");
status = 0; result = GetStatus(status);
Console.WriteLine($"Result: {result}");
status = 10; result = GetStatus(status);
Console.WriteLine($"Result: {result}");
status = -10; result = GetStatus(status);
Console.WriteLine($"Result: {result}");
}
public static string GetStatus(sbyte? status)
{
if (status.HasValue)
{
if(status.Value == 0) { return "Unfinished"; }
if (status.Value == 1) { return "Finished"; }
}
return string.Empty;
}