"Don't say anything to Kateřina!"
Translation:Kateřině nic neříkej!
Our accepted answers in English contain the names both with and without diacritics (Kateřina/Katerina) - this is intentional so that when someone types with plain basic latin letters in English, they don't get the "pay attention" warning.
Why we include these versions in the Czech accepted answers though, that's beyond me.
Anyway, you triggered the warning because you wrote "Kateřine" instead of the correct "Kateřině". There are actually two letters with diacritics in the dative form of the name.