The Communication Widget is not loading due to the JWT token issue.

I am using Java implementation for Jwt token generation. and then providing it inside the communication widget's **authenticate **snippet field. but still, it is not loading. Here is my Java implementation:

private static final String CONNECT_SECRET = "nxt6b+vxK5oXduANxx/xTx8UE3B+68FTwIHZs0Kexxx";
    private static final String WIDGET_ID = "1axa7xc8-93x9-4xd7-96a5-c01x52cxxxxx";
    private static final long JWT_EXP_DELTA_SECONDS = 600;

    public String generateJWT() {
        Map<String, Object> payload = new HashMap<>();
        payload.put("sub", WIDGET_ID);
        payload.put("iat", Instant.now().getEpochSecond());
        payload.put("exp", Instant.now().plusSeconds(JWT_EXP_DELTA_SECONDS).getEpochSecond());

        return Jwts.builder()
                .setHeaderParam("typ", "JWT")
                .setHeaderParam("alg", "HS256")
                .setClaims(payload)
                .signWith(SignatureAlgorithm.HS256, CONNECT_SECRET)
                .compact();

    }

and here is my Javascript implementation:

<script  sec:authorize="isAuthenticated() && (hasRole('ROLE_AGENT') || hasRole('ROLE_BUYER'))">
    let loggedInUsername;
    loggedInUsername = document.getElementById("loggedInUserName").textContent;
    if (loggedInUsername === "") {
        loggedInUsername = "Guest";
    }
    (function (w, d, x, id) {
        s = d.createElement('script');
        s.src = 'https://dtn7rvxwwlhud.cloudfront.net/amazon-connect-chat-interface-client.js';
        s.async = 1;
        s.id = id;
        d.getElementsByTagName('head')[0].appendChild(s);
        w[x] = w[x] || function () {
            (w[x].ac = w[x].ac || []).push(arguments)
        };
    })(window, document, 'amazon_connect', '1axa7xc8-93x9-4xd7-96a5-c01x52cxxxxx');
    amazon_connect('styles', {
        iconType: 'CHAT',
        openChat: {color: '#ffffff', backgroundColor: '#123456'},
        closeChat: {color: '#ffffff', backgroundColor: '#123456'}
    });
    amazon_connect('authenticate', function(callback) {
        window.fetch('/user/generateJwtForConnect').then(res => {
            res.json().then(data => {
                console.log("success", data.message);
                callback(data.message);
            });
        });
    });
    amazon_connect('snippetId', [removed_due_to_long_text]');
    amazon_connect('supportedMessagingContentTypes', ['text/plain', 'text/markdown']);
    amazon_connect('customerDisplayName', function(callback) {
        callback(loggedInUsername);
    });
</script>

the connect secret and widget id are correct. still, it is not loading. instead, it shows that I can not load the chat widget right now.