Problem with Upload to GCS with Typescript (Code is deployed into Lambda)

I want to Upload into GCS using Typescript.
I am using Service Account Json for credentials.

When I tested it it says Uploaded Successfully. But, when I checked the server there was no
file uploaded. Any Idea what this might cause?

import { Storage } from '@google-cloud/storage'
import { SSM } from 'aws-sdk'
import fs from 'fs'
import os from 'os'

export async function uploadFileToGCS(filename: string, bucketName: string): Promise<any> {
  // GCSサービスアカウントの認証情報を指定します。
  const serviceAccountCredentials = await getParameter()
  const credentials = JSON.parse(serviceAccountCredentials)[0]
  const projectId = 'my-project-id'
  const storage = new Storage({
    projectId: projectId,
    credentials: credentials,
  })

  const filePath = os.tmpdir()
  // Create CSV
  fs.writeFileSync(`${filePath}/${filename}`, '', { encoding: 'utf-8' }) // Write the file to the writable directory
  console.log(`CSV file "${filename}" has been created.`)

  try {
    const bucket = storage.bucket(bucketName)

    await bucket.upload(`${filePath}/${filename}`, {
      destination: filename,
    })

    console.log(`File ${filename} uploaded to ${bucketName}.`)

    return Promise.resolve({
      statusCode: 200,
      body: 'Successfully Uploaded',
    })
  } catch (err) {
    console.error('Error uploading file:', err)
    return Promise.resolve({
      statusCode: 500,
      body: 'Failed with Uploading',
    })
  }
}

I am expecting the file to be uploaded into the server.