how to backup whatsapp to icloude when there is not enough space on the iphone

Here is the first step—>Clear Other storage on your iPhone or iPad - Apple Support.

If that doesn’t free enough you can check to offload apps. This will move the app to the cloud, but leave the app data intact. If you tap the app’s icon again it will be reloaded.

You can’t. If you have so little space that WhatsApp cannot stage its backup you are likely to have other problems. Are you sure it is saying that you don’t have enough space on your phone, not that you don’t have enough space in iCloud?

To check iPhone space: Settings/General/iPhone Storage

To check iCloud space: Settings/[your name]/iCloud - Manage Storage

You have a lot of WhatsApp chat. How much storage do you have:

To check iPhone space: Settings/General/iPhone Storage

What are the largest users of storage in that setting?

Good luck! While Apple is a little sheepish about it, corrupted “Other” may be a bug introduced with iOS 13.5 or 13.6. 13.6.1 was released specifically to fix it.