Why can’t iphone 7 users and lower not able to back up the phone?

Hi Brenda_Montoya,

Thanks for posting in Apple Support Communities! If you need more space in iCloud to back up your data, you can upgrade your subscription following these steps:

1. "Go to Settings, then tap your name. 
2. Tap iCloud, then tap Manage Account Storage or Manage Storage. Or scroll down and tap Upgrade to iCloud+.
3. Tap Buy More Storage or Change Storage Plan.
4. Choose a plan and follow the onscreen instructions."

-Upgrade to iCloud+

This will give you more space to back up and store your data. If you don't want to increase your plan, you can Manage your iCloud storage instead to free up space.

Should you be running into an issue with device storage, you may find suggestions to free up space in Settings > General > iPhone Storage. See: How to check the storage on your iPhone and iPad

Hope that helps!