Unable backup my iPhone data on to iCloud.

It's possible that the size of your next backup exceeds the space available in iCloud.

Navigate to settings > Apple ID > iCloud > iCloud backup > {device name}

The next backup size will display after calculating.

How to back up your iPhone, iPad, and iPod touch - Apple Support

AlWeir wrote:

iCloud isn't a backup solution, is a synchronization service. If you want a backup solution, then I suggest you look at GoogleDrive, OneDrive or Mega.

AIWeir, You have posted this several times in other threads, and it is WRONG, and has been WRONG every time you have posted it.

From Apple’s Knowledge Base→How to back up your iPhone or iPad with iCloud - Apple Support. Do you think Apple would have provided instructions to back up to iCloud if it wasn’t a backup solution?

iCloud+ has 2 independent functions. iCloud backups, and iCloud sync. It’s confusing because they both have the same name prefix. But they have no connection with each other. Go to Settings/[your name]/iCloud. You will see a bunch of switches (with iOS 16 or later also tap Show All). When you turn on a switch that data type will sync to iCloud. If you do this on multiple devices that share an Apple ID the selected data will sync to all of those devices. 

You can also turn on iCloud Backups. This will back up your phone every night if the phone is plugged in, connected to Wi-Fi and locked.

AlWeir wrote:

iCloud isn't a backup solution, is a synchronization service. If you want a backup solution, then I suggest you look at GoogleDrive, OneDrive or Mega.

Yes, iCloud IS a backup solution for iPhones and iPads.

iCloud isn't a backup solution, is a synchronization service. If you want a backup solution, then I suggest you look at GoogleDrive, OneDrive or Mega.