I can't back up to iCloud.

On my friend's phone (iPhone 14) the message appeared: Unable to back up your device. In storage it shows that there is not enough space and that I need to upgrade to iCloud. I don't want to upgrade. I want to remove WhatsApp from the backup (it takes up 4.2 GB) and then there is enough space left for the backup. If I go to Storage and click on WhatsApp and then on delete data from iCloud, it tells me: "This action will delete all WhatsApp data stored in iCloud and on any devices that sync with iCloud. It also says to disable syncing for WhatsApp on all devices to prevent WhatsApp from storing additional data in iCloud." I just want to be able to back up again, without upgrading and without deleting the WhatsApp content from my phone. I will back up to iCloud with the other applications that don't take up more than 5 GB. Please help me and explain exactly what I need to do. I'm not very good at technology. Thank you!