How do I add new, writeable files to a Wordpress child theme with the right permissions?

I’m trying to add a new Wordpress child theme file and directory and seem to be running into some permission issues.

Using the DigitalOcean FTP Guide, I was able to add the new child theme file (inside a directory, to mirror the parent theme). However, I’m not able to edit that file inside the Wordpress dashboard, and it gives me the following error:

You need to make this file writable before you can save your changes.

Looking at the directory, it appears that this new directory has both user and group permissions set to root, while all other files and directories are set to www-data.

I tried to set permissions based on this post using the following commands:

chown -R www-data:www-data /var/www/
chmod -R 766 /var/www/

However, I received the following errors:

Invalid flag -R

and when that character was removed:

You must supply a numeric argument to the chown command.

I also tried using sudo, but received the following response:

Invalid command.

Would love any help!