Hey Justin,
Did you do full migration from Atlas 2 to 3 following Mendix docs?
https://docs.mendix.com/refguide9/moving-from-atlas-2-to-3/
If yes you probably made some mistake because your path should be different (on error)
e.g. http://localhost:8085/deployment/theme-cache/styles/web/sass/app/_Diagram.scss something like this