How to back up WhatsApp messages on iPhone 11 with sufficient storage?

Hello so i have an iPhone 11 and my last whatsapp backup was on Oct 2023 (because of storage issues)...but i recently tried to back up my whatsapp with my current messages on the iPhone 11 but the message "free up -20, 388, 780 KB of storage space on your iPhone to back up your chats" kept appearing even though i do have 20GB free space for it to back up...

I even backed up my iCloud to restart and reset all my settings (to see if the problem disappeared) but it is still the same....

What should i do more to resolve this issue so that when i get my new iphone my whatsapp is backed up?

[Re-Titled by Moderator]

Original Title: iPhone 11