Just to clarify, when you say:
sftp by disabling Password Authentication and getting access via password
What do you mean? You’ve disabled SSH and now use password authentication, or in filezilla you now have a proper SSH key and you don’t need to enter a password?
Now have i am not able to upload or write data getting Permission issue on write and delete on www-data
I’m thinking by this you are sftp via the ubuntu user and you don’t have write permissions to say, /var/www or directories owned by www-data?
You’ll need to fix up your ownership and permissions to solve. Depending on how you want your server to operate there are many options. Will you be the only one uploading files, or do you have multiple developers? Generally you don’t want to be using the www-data user for sftp / ftp access - you could grant another user group permissions to the www-data group and then that user can write (if you set the right sticky bits and top level permissions) but if you actually want to do that depends on how you plan to operate this server.
We have used a combination of symlinks for users with their own SSH / credentials, sticky bits, etc depending on if we are on the dev or production servers. We also have jobs which run that lock the file permissions down so that nobody can change them, when we want to update things we run another job that chowns back to www-data; but we’re far more locked down that your typical EE user.