400MB free ram but cant delete WhatsApp chats
I am not responsible for lost data
Try to do this:


Uninstall whatsapp
Search in internal storage the whatsapp folder
Delete it
Reboot your phone
Reinstall whatsapp and DON'T restore backups


If this procedure doesn't work, I think it is a good idea resetting your phone.
Unfortunately devices like your have this type of problems frow the dawn of the times and the only solution often is the total wipe