Please, check the SASS course for bugs, a month ago we were discussing this issue: https://discuss.codecademy.com/t/arithmetic-functions-scss/77315
I' ve tried and tried again, refreshed page, disabled adblocker and trackersblocker, tried with several browsers: chrome, opera, IE, firefox on linux, no way.
So it does not depend on browser, scripts and OS, it is just a codecademy's own bug.
Not possible to solve, not possible to continue with the other exercises and complete the course.
And to know, one of the main reasons why I cancelled my subscription to pro.