In Coda in the menu "Text > Syntax Mode > Sass" is just for .SCSS-syntax. I have been working with .SCSS a while now and would like to try .SASS-syntax.
After searching around I didn't find a solution for this, also not here on SO (this question is dated and the answers are not really correct or what I was looking for)
I found a project on Github which solves this issue. Follow the instructions written in the repository and you are ready to go with .sass files.
Here is the link.