Hello ineedhelpwithmystorage,
Thank you for using Apple Support Communities! I understand that the Next Backup Size on your iPhone exceeds the amount of available storage in iCloud. It's important that you're able to preserve your data by making a backup, and I would be happy to help.
The following support article provides some additional suggestions for the scenario that you're encountering:
If you can't back up to iCloud or restore from iCloud backup failed - Apple Support
Specifically, check out this section:
"After you check to see how much iCloud Storage you have, try these steps to complete your backup:
- Upgrade your iCloud storage plan.
- Reduce the size of your backup.
- Delete any data that you don't need.
- If you still need help, contact Apple Support."
Another option would be to make an iTunes backup instead:
How to back up your iPhone, iPad, and iPod touch - Apple Support
Take care!