Error Compiling Autorest Generated Client

After autogenerating client using Swagger OpenApi 3.0 spec, I'm unable to compile the generated client. I get error on objects ClientDiagnostics, RawRequestUriBuilder, Utf8JsonRequestContent.

RestClient.cs

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

// <auto-generated/>

#nullable disable

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Threading;
using System.Threading.Tasks;
using Azure;
using Azure.Core;
using Azure.Core.Pipeline;
using Planner.Client.Models;

namespace Planner.Client
{
    internal partial class RestClient
    {
        private Uri endpoint;
        private ClientDiagnostics _clientDiagnostics;
        private HttpPipeline _pipeline;

        /// <summary> Initializes a new instance of RestClient. </summary>
        /// <param name="clientDiagnostics"> The handler for diagnostic messaging in the client. </param>
        /// <param name="pipeline"> The HTTP pipeline for sending and receiving REST requests and responses. </param>
        /// <param name="endpoint"> server parameter. </param>
        public RestClient(ClientDiagnostics clientDiagnostics, HttpPipeline pipeline, Uri endpoint = null)
        {
            endpoint ??= new Uri("");

            this.endpoint = endpoint;
            _clientDiagnostics = clientDiagnostics;
            _pipeline = pipeline;
        }


        internal HttpMessage CreateCreateDataRequest(IEnumerable<DataCreateModel> body)
        {
            var message = _pipeline.CreateMessage();
            var request = message.Request;
            request.Method = RequestMethod.Post;
            var uri = new RawRequestUriBuilder();
            uri.Reset(endpoint);
            uri.AppendPath("/api/Budget/CreateData", false);
            request.Uri = uri;
            if (body != null)
            {
                request.Headers.Add("Content-Type", "application/json");
                var content = new Utf8JsonRequestContent();
                content.JsonWriter.WriteStartArray();
                foreach (var item in body)
                {
                    content.JsonWriter.WriteObjectValue(item);
                }
                content.JsonWriter.WriteEndArray();
                request.Content = content;
            }
            return message;
        }
    }
}

Also, getting error in below autogenerated serialization class on IUtf8JsonSerializable and Optional.

DataCreateModel.Serialization.cs

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

// <auto-generated/>

#nullable disable

using System.Text.Json;
using Azure.Core;

namespace Planner.Client.Models
{
    public partial class DataCreateModel : IUtf8JsonSerializable
    {
        void IUtf8JsonSerializable.Write(Utf8JsonWriter writer)
        {
            writer.WriteStartObject();
            if (Optional.IsDefined(ProjectCode))
            {
                if (ProjectCode != null)
                {
                    writer.WritePropertyName("projectCode");
                    writer.WriteStringValue(ProjectCode);
                }
                else
                {
                    writer.WriteNull("projectCode");
                }
            }
            if (Optional.IsDefined(TeamCode))
            {
                if (TeamCode != null)
                {
                    writer.WritePropertyName("teamCode");
                    writer.WriteStringValue(TeamCode);
                }
                else
                {
                    writer.WriteNull("teamCode");
                }
            }
            if (Optional.IsDefined(YearMonth))
            {
                writer.WritePropertyName("yearMonth");
                writer.WriteNumberValue(YearMonth.Value);
            }
            if (Optional.IsDefined(Hours))
            {
                writer.WritePropertyName("hours");
                writer.WriteNumberValue(Hours.Value);
            }
            writer.WriteEndObject();
        }
    }
}

This project using .net 5, autorest v3.1.0, cli v3.1.2 and node v14.15.5.

What am I missing?