Issues trying to create new error types

c#
public class RFC8807Error: Exception
{
    public required virtual string type { get; init; }
    public required virtual string title { get; init; }
    public virtual int status { get; init; } = 400;
    public required virtual string detail { get; init; }
    public virtual string? instance { get; init; }

    private JsonSerializerOptions jsonOpts = new() { DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull };
    public string toJsonError() => JsonSerializer.Serialize(this, jsonOpts);
}

c#
public class RFC8807Error: Exception
{
    public required virtual string type { get; init; }
    public required virtual string title { get; init; }
    public virtual int status { get; init; } = 400;
    public required virtual string detail { get; init; }
    public virtual string? instance { get; init; }

    private JsonSerializerOptions jsonOpts = new() { DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull };
    public string toJsonError() => JsonSerializer.Serialize(this, jsonOpts);
}

c#
public class unknownCapabilityException : RFC8807Error
{
    public unknownCapabilityException()
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
    }
}

c#
public class unknownCapabilityException : RFC8807Error
{
    public unknownCapabilityException()
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
    }
}

c#
string errorDetail = $"JMAP requires POST, for all requests. Verb {req.Request.Method} is not supported.";
var errorValue = new unknownCapabilityException()
{
    detail = errorDetail,
    status = 405,
};
string errorJson = errorValue.toJsonError();

// ... Send the error JSON file back to the client later

c#
string errorDetail = $"JMAP requires POST, for all requests. Verb {req.Request.Method} is not supported.";
var errorValue = new unknownCapabilityException()
{
    detail = errorDetail,
    status = 405,
};
string errorJson = errorValue.toJsonError();

// ... Send the error JSON file back to the client later

   public unknownCapabilityException(string detail, int status)
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
    }

   public unknownCapabilityException(string detail, int status)
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
    }

c#

public class unknownCapabilityException : RFC8807Error
{
    public unknownCapabilityException(string detail, int status = 400)
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
        base.detail = detail;
        base.status = status;
    }
}

...

var errorValue = new unknownCapabilityException(errorDetail, 405);

c#

public class unknownCapabilityException : RFC8807Error
{
    public unknownCapabilityException(string detail, int status = 400)
    {
        base.type = "urn:ietf:params:jmap:error:unknownCapability";
        base.title = "Unknown Capability";
        base.detail = detail;
        base.status = status;
    }
}

...

var errorValue = new unknownCapabilityException(errorDetail, 405);