Implementing web token access in spring boot

In this i tried to implement to render i-frame without login . 
Below i've attached my backend code for jwt token generation too.

@Service
public class SisenseTokenService {

// Use the Shared Secret from your Sisense Admin panel
private static final String SHARED_SECRET = "MY_secret_key";

public static String generateSisenseJWT(String userEmail) {
long nowEpoch = Instant.now().getEpochSecond();

return Jwts.builder()
.claim("sub", userEmail) // Sisense user email
.claim("iat", nowEpoch) // Issued At
.claim("jti", UUID.randomUUID().toString()) // Unique token ID
.signWith(SignatureAlgorithm.HS256, SHARED_SECRET.getBytes(StandardCharsets.UTF_8))
.compact();
}

}

output for token:

token:e

decoded:
{
  "sub": "[email protected]",
  "tenantId": "system",
  "iat": ,
  "jti": ""
}
in frontend - 

<iframe  src={`https://sisense-dev.cscs-apps.com/app/main/dashboards/686bca7cef95f60033490c63?embed=true&web_access_token=e`}

                width="100%"

                height="100%"

            />

still it is not working why? kindly help asap, it is an important blocker