can I use iCloud for time machine backups

As per the article Other ways to back up and restore files - Apple Support

Use cloud-based services

Cloud-based services don't require you to connect an external hard drive to your Mac. For example, iCloudcan automatically store your photos, calendars, contacts, notes, passwords, and more. And it includes iCloud Drive, which can store documents, desktop files, and other files of your choosing. To access those files, you just need to be signed in to iCloud. Other cloud-based services are available from third parties.

You can't use iCloud for time Machine backups , time Machine back up the data see the article Back up your Mac with Time Machine - Apple Support

Use Time Machine, the built-in backup feature of your Mac, to automatically back up your personal data, including apps, music, photos, email, and documents. Having a backup allows you to recover files that you later delete or can't access. Learn how to restore your Mac from a backup.