I think this issue is related to the changes Netlify made mentioned in Project that's been stable for years has suddenly started failing "poetry: command not found"
I ran into the same issue for my mkdocs site, however I was able to use the workaround mentioned in the other thread because I’m using a netlify.toml /poetry for my build settings. Unfortunately I’m not sure how to remedy this in the UI build settings.
Tagging the Netlify folks who replied in the other thread for visibility: @serhalp @lukas.holzer