Okay, first and most importantly:
Error messages about main.css while running yarn start (or yarn run start) are normal, expected, and by design.
This happens because main.css actually doesn’t exist while running yarn start. Instead, styles are built in memory and injected into the page by Webpack. Errors related to main.css in the browser console can and should be safely ignored while running yarn start.
If you are running yarn start and your page/site looks unstyled in the browser
This can happen sometimes. It’s inconsistent and seems to have something to do with the imperfect software that “shares” your local theme development directory with the Vagrant box. You can fix it either by making a change to, and saving, any .scss file, or hard-refreshing the page (with SHIFT+Refresh), or sometimes both.
If the missing main.css file message persists after running yarn build or yarn build:production
Make sure you’re not refreshing your page WHILE the yarn task is running. Sage versions its assets and browsers are really reluctant to give up on caches of stylesheets. Let yarn finish its task, wait a beat, and then refresh your browser.
Let us know if any of that helps.