how is 'františek is not saying that word' incorrect?
Gramatically the translation is IMHO correct. The meaning might sound strange.
Is "František can't say that word" incorrect? I didn't report it because I'm not sure if I'm correct or not.
That is "František to slovo nemůže říct.". Can is a modal verb.
Could it also be "František don't say that word"? Meaning that I am telling him to not say a word.
No, the imperative mood is done differently and will be the topic of some future skill. It is actually "Františku, to slovo neříkej!".