I have purchased 50gb icloud it says i have used only9.9gb of it. But still wen i back up it says no enough storage… what can be done to this

You need to check how large the backup you are trying to make is. if it is larger than 40.1 GB you dont have enough space for it.

See:

If you can't back up to iCloud - Apple Support

Check your iCloud storage and Next Backup Size in iOS or iPadOS

1. Go to Settings > [your name].
2. Tap iCloud. The amount of iCloud storage that you've used is listed at the top. Learn more. 
3. Tap Manage Storage.
4. Tap Backups, then tap the device you're using. Next Backup Size is listed under Last Backup and Backup Size. 

Good luck!

That would mean the backup would require more than 40.1GB. How much storage is used on your phone?