这可能是由于缺少身份验证所致。需要在POSTMAN中设置正确的Authorization头和签名以进行验证。

具体步骤如下：

1. 在API Gateway中创建API并启用API密钥，以便进行身份验证。
2. 在POSTMAN中打开要调用的API，并在Authorization标头中填写API密钥。
3. 根据API Gateway提供的签名算法计算签名并在POSTMAN中填写签名。

示例代码如下：

var CryptoJS = require("crypto-js");
var date = new Date();
var timestamp = date.getTime().toString().slice(0,-3);
var secretKey = 'YOUR_SECRET_KEY';
var region = 'YOUR_REGION';
var serviceName = 'apigateway';
var httpMethod = 'POST';
var canonicalUri = '/path/to/your/api';
var canonicalQueryString = '';
var canonicalHeaders = 'content-type:application/json\nhost:YOUR_API_GATEWAY_ENDPOINT\nx-amz-date:'+timestamp+'\nx-amz-target:YOUR_API_GATEWAY_ACTION';
var payload = '{"param1": "value1", "param2": "value2"}';
var hashedPayload = CryptoJS.SHA256(payload).toString(CryptoJS.enc.Hex);
var canonicalSignedHeaders = 'content-type;host;x-amz-date;x-amz-target';
var canonicalRequest = httpMethod+'\n'+canonicalUri+'\n'+canonicalQueryString+'\n'+canonicalHeaders+'\n'+canonicalSignedHeaders+'\n'+hashedPayload;

var algorithm = 'AWS4-HMAC-SHA256';
var credentialScope = date.toISOString().slice(0,10)+'/'+region+'/'+serviceName+'/aws4_request';
var stringToSign = algorithm+'\n'+timestamp+'\n'+credentialScope+'\n'+CryptoJS.SHA256(canonicalRequest).toString(CryptoJS.enc.Hex);
var signingKey = CryptoJS.HmacSHA256(date.toISOString().slice(0,10), 'AWS4'+secretKey).toString(CryptoJS.enc.Hex);
signingKey = CryptoJS.HmacSHA256(region, signingKey).toString(CryptoJS.enc.Hex);
signingKey = CryptoJS.HmacSHA256(serviceName, signingKey).toString(CryptoJS.enc.Hex);
signingKey = CryptoJS.HmacSHA256('aws4_request', signingKey).toString(CryptoJS.enc.Hex);
var signature = CryptoJS.HmacSHA256(stringToSign, signingKey).toString(CryptoJS.enc.Hex);

pm.collectionVariables.set('timestamp', timestamp);
pm.collectionVariables.set('signature', signature);

pm.sendRequest({
    url: 'https://YOUR_API_GATEWAY_ENDPOINT'+canonicalUri+canonicalQueryString,
    method: httpMethod,
    header: {
        'content-type': 'application/json',
        'host': 'YOUR_API_GATEWAY_ENDPOINT',
        'x-amz-date': '{{timestamp}}',
        'x-amz-target': 'YOUR_API_GATEWAY_ACTION',
        'Authorization': algorithm+' Credential='+secretKey+'/'+credentialScope+', SignedHeaders='+canonicalSignedHeaders+', Signature={{signature}}'
    },
    body: {
        mode: 'raw',
        raw: payload
    }
}, function(err, res) {
    console.log(res.body);
    pm.environment.set('response', res.body);
});