Want to icloud backup my wifes ipad, got enough storage, get failed message again and again, help

If you are relying upon your Wife’s [paid] 200GB iCloud storage quota, you need to ensure that you have correctly configured Family Sharing - and that both of you are members of the same Family group.

More information can be found here:

Family Sharing - Apple

Set up Family Sharing – Apple Support

Share App Store, iTunes Store and Apple Books purchases with Family Sharing – Apple Support

https://www.imore.com/how-set-up-family-sharing-iphone-ipad

Have you tried to back the iPad to a Mac or a Windows machine? If you succeed, I strongly suggest your wipe and reset the iPad.

How to back up your iPhone, iPad, and iPod touch

https://support.apple.com/en-us/HT203977

I'm not an expert on this, so I may be wrong, but it seems to me that your icloud storage is registered with your Apple ID. If your wife has her own Apple ID, it wouldn't be available to her - she needs to use her own icloud account.