Call AWS-SageMaker API in flutter

Question

I'm trying to call a SageMaker API in flutter but the header has some issues related to Signature.

Map<String, String> signRequest(String param) {
var method = "POST";
var uri = endpoint;
var secretKey = secretKey;
var accessKey = accessKey;
var region = awsRegion;
var service = serviceType;
var host = baseUrl;

var date = DateFormat("yyyyMMdd'T'HHmmss'Z'").format(DateTime.now().toUtc());
var date2 = DateFormat("yyyyMMdd").format(DateTime.now().toUtc());

var requestBody = utf8.encode(json.encode(param.toLowerCase()));
var hashedPayloads = sha256.convert(requestBody).toString().toLowerCase();

var canonicalUri = uri;
var canonicalQuerystring = "";
var canonicalHeaders =
    "content-type:application/json\nhost:$host\nx-amz-content-sha256:$hashedPayloads\nx-amz-date:$date\n";

var signedHeaders = "content-type;host;x-amz-content-sha256;x-amz-date";
var canonicalRequest =
    "$method\n$canonicalUri\n$canonicalQuerystring\n$canonicalHeaders\n$signedHeaders\n$hashedPayloads";

var credentialScope = "$date2/$region/$service/aws4_request";

var stringToSign =
    "${ApiConstants.hmacShaTypeString}\n$date\n$credentialScope\n${sha256.convert(utf8.encode(canonicalRequest))}";

var kSecret = "AWS4$secretKey";
var kDate = Hmac(sha256, utf8.encode(kSecret))
    .convert(utf8.encode(date2))
    .toString();
var kRegion = Hmac(sha256, utf8.encode(kDate))
    .convert(utf8.encode(region))
    .toString();
var kService = Hmac(sha256, utf8.encode(kRegion))
    .convert(utf8.encode(service))
    .toString();
var kSigning = Hmac(sha256, utf8.encode(kService))
    .convert(utf8.encode("aws4_request"))
    .toString();
var signature = Hmac(sha256, utf8.encode(kSigning))
    .convert(utf8.encode(stringToSign))
    .toString();

var authorizationHeader =
    "${ApiConstants.hmacShaTypeString} Credential=$accessKey/$credentialScope, SignedHeaders=$signedHeaders, Signature=$signature";


Map<String, String> headers = {
  "Content-Type": "application/json",
  "X-Amz-Date": date,
  "X-Amz-Content-Sha256": hashedPayloads,
  "Authorization": authorizationHeader
};
return headers;

}

Status code: 403

Got this message in response:

{"message":"The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.\n\nThe Canonical String for this request should have been"}