Why can't I backup my iPhone data to iCloud despite having plenty of storage?

You have confused iCloud and System storage together, which are two very different things that cannot be substitutions for each other. iCloud Storage merely holds your backups, syncs data, and some data including Photos, Messages, and other of Apple’s proprietary data. But your system storage is what holds all of your apps and data with them. The key difference is iCloud storage can be increased, while system storage cannot and is fixed with the purchase of your device.

Your iPhone’s system storage needs temporary space to prepare the backup and upload it to iCloud, if it is low on free storage it cannot do so. If you open Settings > [your name] > iCloud > iCloud Backup > Select your device > There will load how much storage the next backup needs. You need to start deleting apps and data you don’t need. If you can’t, you’ll be better off purchasing a new iPhone with more storage.

Read this: What's the difference between device storage and iCloud storage? - Apple Support

alishba163 wrote:

hey first if all tysm for ur response, it really helped. now what i wanna ask next is that if delete photos from my gallery, will those be deleted from my icloud storage aswell?

Yes. By the way, you will find that videos take up substantially more storage space so deleting those will help.