Based on the issue you’ve reported and preview of the shell bash, it looks as if you may have the wrong owner/group and permissions set for the website’s docroot.
More than likely in your hosting environment, the entire ks320 directory currently does not have www-data as the group assignment.
It can be owned by root but should be assigned to the www-data group (recursively). Running an ls -l from the /var/www directory will output the current permissions, owner, and group assignments to each file and folder.
It probably resembles
drwr-xr-x ### root root ### DATE FILE_OR_FOLDER_NAME
but should resemble:
drwrwxr-x ### root www-data ### DATE FILE_OR_FOLDER_NAME
You can accomplish changing the group assignment from the /var/www directory with the following command:
chown -R root:www-data ks320
Then, you’ll also want to make sure the group has read/write/execute permissions on the directory. You can accomplish this from the /var/www directory with:
chmod -R 775 ks320
Once the permissions and group assignments to the folders have been set, your “www-data” user should be able to create the plugin directory and unzip the wordpress plugin files into place.