If you don't know if you should use 'ss' or 'ß', stick with 'ss.' Duolingo may point it out to you that you should have used 'ß' but it is never actually wrong to use 'ss' instead. For example, if someone in Germany and wrote 'Grossvater,' it would be fine. In fact, ß is being slowly eliminated from the language. It is no longer used in Swiss German, and I think it has been eliminated in Austrian German as well. Damn shame.
Switzerland and Liechtenstein stopped teaching the use of ‘ß’ in the 1930s, but it is definitely still taught and used in Austria. The use of ‘ß’ declined somewhat with the introduction of the first typewriters, some of which lacked the letter; and again with the first computers. But ‘ß’ is now making a comeback in Switzerland and Liechtenstein with the prevalence of autocompletion and autocorrection software; and in SMS text and chat, because ‘ß’ is shorter than ‘ss’.
It depends on the case and the gender.
This sentence makes me think of Heidi every time. So I went looking for an untranslated version and found this, if anyone wants to try their hand at reading a longer German text: http://www.gutenberg.org/files/7500/7500-h/7500-h.htm