why does my iphone say that i don't have enough space to back up on icloud when i do have space

It sounds like the next backup of your iPhone would be larger than the <5 GB of space you have available in iCloud. Here's how you can check how large the next backup of your iPhone would be:

1. In iOS, go to Settings > [your name].
2. Tap iCloud. At the top, you can check how much iCloud storage you've used.
3. Tap Manage Account Storage or Manage Storage.
4. Tap Backups, then tap the device you're using. Under Last Backup and Backup Size, you can check Next Backup Size. It may take a few minutes for your device to calculate Next Backup Size. 
5. If your backup needs more space than the amount you have available in iCloud, you can: 

• • Delete information that you don't need to reduce the size of your backup
  • Upgrade your iCloud+ plan to get more space

If you still need help, contact Apple Support.

This was helpful