Hi there IndusWebi.com,
Thanks for using Apple Support Communities. It looks like you are not able to back up your WhatsApp chats to iCloud and it is saying you need additional storage on the iPhone to turn on iCloud backup. You may need additional storage to complete a backup in either iCloud or in the iPhone itself. According to WhatApp's FAQ page regarding backups "There must be enough free space on both your iCloud and iPhone. You need to have at least 2.05 times the space available in your iCloud account and on your phone than the actual size of your backup." Learn more here: How to back up to iCloud.
These articles may help:
How to check the storage on your iPhone, iPad, and iPod touch
If you clear more space and continue to have an issue, reach out to WhatsApp developers for more help: How to contact an app developer.
Have a great day!