Directories not created

I just installed WPPA and it looks fantastic. But I am having a few problems with directories not being created.

My first encounter was the album admin page, where there is no thumbnail. See this screenshot:

https://share.getcloudapp.com/7KuxR5RR

I tried adding the folder

/wp-content/wppa-pl/

and uploaded another picture to the album. However, the thumbnail doesn’t show up for the new picture either.

Then I tried the basic shortcode for a slide show. For example, see this page:

https://mpa.atticsofmylife.org/father/

The slideshow is working fine, using this shortcode:

[wppa type=”slide” album=”3″ align=”center”]

But the filmstrip (or whatever it is) below that, which is scrolling automatically, is just a bunch of broken images.

Apparently this directly was not created:

/wp-content/uploads/sites/2/wppa/thumbs/

Can you help me figure out how to resolve this issue? I’m assuming there was some sort of permission issue on installation.

1) this is a multisite installation, and it uses subdomains (not subdirectories)
2) I added the constant to wp-config as indicated for a “separate albums for each blog” installation
3) I got one warning message at the beginning about a folder that had to be manually created and given permissions. I did that and the warning went away, so I assumed I had done it correctly:

/wp-content/uploads/sites/2/wppa/
set permissions to 775

Thank you for any advice!