UnhandledPromiseRejectionWarning on Request to Pixellena Lux API

I got the following error while trying to convert a jpeg image to webp by using Pixellena Lux API.

(node:13962) UnhandledPromiseRejectionWarning: #<RequestIssueException>
(Use `node --trace-warnings ...` to show where the warning was created)
(node:13962) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:13962) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

Documentation of API.

https://pixellena.com/docs/sdk/

I just updated the following part as

let adjustments = {
    "shifter": {
        "steps": [
            {
                "scale-to": {
                    "width": 10
                }
            }
        ]
    },
    "encoder": {
        "quality-measure": "fsim-c",
        "qual-threshold": 2.0
    }
}