Unable to move iPhone SE data to iCloud when storage is full

iCloud is not additional storage for your iPhone. iCloud is a synchronization service to synch content across all your Mac devices. If your iPhone is full you can reduce the photos' size by turning on optimization. This puts the full size photos in iCloud and reduces the size of the photos on your device until you need them.

See here.

https://support.apple.com/en-us/105061

Having said that, 64MB memory is a very very small amount of memory by today's standards and you are going to struggle with it no matter what you do.

64 GB is fine for home button phones, I use two, the iOS and System files are smaller usually, 15 and 8GB respectively

I would recommend making sure that

1. you have 200GB iCloud storage, Only £€$2.99 a month
2. Everything, yes everything is sync'd to iCloud in Settings, iCloud
3. Having ensured that the sync is complete, wait maybe 2 days, you then
4. Backup to iCloud (which will be smaller size now)
5. Erase all content and settings
6. Restore the backup - this will reduce system files to nearer 6-8GB and reestablish Photos with thumbnails

Research the instructions, it has some risks. Whatsapp may need additional internal backup.

Note that if you let your phone's built-in (device) storage become completely full, this might interfere with normal operation – including attempts to back up or synchronize data to iCloud.

You always want some free device storage at all times, and may need more when updating iOS.

As others are explaining, iCloud is not an archival system. It is a synchronization system. You can't offload data to the cloud and then remove it from your phone as in real time, when you delete things you "sync" with iCloud, they will be removed everywhere, including from iCloud.

This link explains the difference between device and iCloud Storage --> https://support.apple.com/en-us/102670

When people buy iPhones, storage is one of the most overlooked considerations. And the simple fact is buying an iPhone with the lowest storage option can be a mistake as you're now learning. Once purchased, there is no way to add storage to an iPhone. This means if you own an iPhone with not enough storage to meet your needs, you're left with constantly having to delete things you don't need or use. This can include: Apps, Music, Files, Photos, Videos, Messages, Email, etc.

Today, new iPhones sold by Apple start at 256GB. Even that in today's modern world may not be enough. I buy my new iPhones with 512GB. Our storage needs will only grow, not shrink. No one has ever come here complaining they have too much storage. But daily people come here upset because they don't have enough.

Next time you buy an iPhone, give more consideration to how much storage you get and make sure to buy more than you think you may need.

Your iPhone's physical storage (64GB) and your iCloud storage are two separate spaces.

• iPhone storage is the fixed hardware chip inside your phone that holds iOS, active apps, local app caches, and downloaded files.
• iCloud storage is online cloud space on Apple's remote servers used for backups, file syncing, and sharing data across devices. 

Paying for or having extra iCloud storage does not physically expand the 64GB capacity built into your iPhone. 

Out of interest, what is the size of the System Data? That along with Whatsapp is a likely candidate, but apps like YouTube can be temporarily uninstalled with no problems reinstalling.

I see you are still on iOS18 for some reason, those little rockets will run iOS27 just fine, though the currect spontaneous Silent setting may mean its better to stave off for now.

If you sync everything and backup Whatsapp internally like I suggested then you can always un sync photos and delete apps like Whatsapp in order to update iOS. I had to do that for installing iOS version that we are not allowed to talk about.