I know you said upgrading to Mendix 9 isn’t an option, but if you can upgrade your application to Mendix 9 or higher then you can edit styling directly in Studio Pro without the need to install and run Calypso. It’s a lot easier as changes to styles are reflected automatically as you build.
If you really can’t upgrade, then make sure you have Calypso running before you make any changes to the .scss file. If you have Calypso setup to watch the correct location it should detect when you save your changes and build the .css file. You can check this in Calypso’s console. If it doesn’t build, you probably don’t have Calpyso setup correctly.
did you try and follow the manual? https://docs.mendix.com/howto8/front-end/calypso/