From the guide:
Why are some of the pictures in webp format?
"It is a format that allows more detail and sharper pictures when the developer does the renders without increasing the photos to a ridiculous size of bytes. Windows can display them if you download the coded pack. After that you can open them in almost any application [...]
Codec is here:
You must be registered to see the links
bpCodecSetup.exe
You will need to restart Windows for the Explorer preview to work after you've installed the codec."
If you haven't downloaded the Webp codec, you're going to get errors when your comp/Renpy tries to access Webp files. Download the codec and try to access the scene again. If you still can't get it--or you've already downloaded the codec--then there's either an issue with your download or a bug that needs to be reported on the Patreon.