WordPress plugin unable to write to folder

I set up WordPress on a Ubuntu server and used this tutorial to add extra security https://www.digitalocean.com/community/tutorials/how-to-configure-secure-updates-and-installations-in-wordpress-on-ubuntu

I’m having a hard time with some plugins saying they can’t write to folders. I’m trying to install a backup plugin and it says it can’t write to the backups folder even though it is showing as 755.

Here’s what I have for ownership - I think I may have my owners and groups in a muddle.

Can anyone help me make sure they are set up properly?

/var/www/html/wp-content/backups$ ls -alh drwxr-sr-x 2 wp-user www-data 4.0K Aug 9 11:42 . drwxr-xr-x 12 wp-user wp-user 4.0K Aug 30 08:35 … -rw-rw-r-- 1 wp-user wp-user 28 Mar 16 04:01 index.php