Authorization Header Requires 'Credential' Parameter

We are using Identity Server4 with .NET Core and deploy the application as AWS Serverless lambda function. When are calling the token endpoint to generated access token we got the following error message:

{
"message": "Authorization header requires 'Credential' parameter. Authorization header requires 'Signature' parameter. Authorization header requires 'SignedHeaders' parameter. Authorization header requires existence of either a 'X-Amz-Date' or a 'Date' header. Authorization=Basic Y2xpZW50OnNlY3JldA=="

}

Here is our ConfigurationServices method in Identity Server application:

 public void ConfigureServices(IServiceCollection services)
    {
        services.AddSingleton<IConfiguration>(Configuration);

        //connection string
        string connectionString = Configuration.GetConnectionString("IdentityServer");

        var rsaProvider = new RSACryptoServiceProvider(2048);

        SecurityKey key = new RsaSecurityKey(rsaProvider);

        var credentials = new Microsoft.IdentityModel.Tokens.SigningCredentials
              (key, SecurityAlgorithms.RsaSha256Signature);


        var migrationsAssembly = typeof(Startup).GetTypeInfo().Assembly.GetName().Name;

        services.AddIdentityServer()
           .AddSigningCredential(credentials)
            // this adds the config data from DB (clients, resources)
            .AddConfigurationStore(options =>
            {
                options.ConfigureDbContext = builder =>
                builder.UseSqlServer(connectionString,
                sql => sql.MigrationsAssembly(migrationsAssembly));
            }) // this adds the operational data from DB (codes, tokens, consents)
            .AddOperationalStore(options =>
            {
                options.ConfigureDbContext = builder =>
                builder.UseSqlServer(connectionString,
            sql => sql.MigrationsAssembly(migrationsAssembly));

                // this enables automatic token cleanup. this is optional.
                 options.EnableTokenCleanup = true;
                 options.TokenCleanupInterval = 30;
            });

        // Add S3 to the ASP.NET Core dependency injection framework.
        services.AddAWSService<Amazon.S3.IAmazonS3>();
    }

Here is our client application that calling identity server's token endpoint to generate token:

[HttpGet]
    public async Task<IActionResult> Get(string client, string secret)
    {

        IActionResult result = null;

        //discover endpoints from metadata

        //var disco = await DiscoveryClient.GetAsync("");

        var disco = await DiscoveryClient.GetAsync("hide for security reasons/");

        if (disco.IsError)
        {
            result = NotFound(disco.Error);

            return result;
        }
        //request token

        var tokenClient = new TokenClient(disco.TokenEndpoint, client, secret);

        var tokenResponse = await tokenClient.RequestClientCredentialsAsync(scope: "sup");

        if (tokenResponse.IsError)
        {
            result = NotFound(tokenResponse.Error);
        }

        result = Ok(tokenResponse.Json);

        return result;
    }