"Kateřina slowly closed her eyes."
Translation:Kateřina pomalu zavřela oči.
The Czech word order is very free but each variation often caries a subtle difference in the meaning and often has to be pronounced with some special intonation. That is the danger here, we have to accept many forms, but many are not the default and the student can't see that when his/her answer is accepted.
Usually it depends on what is your actual message. What is the topic and what is the comment https://en.wikipedia.org/wiki/Topic_and_comment
In this case the Czech and the English follow each other quite closely. The main new information here is that she closed her eyes and as an additional information we have that she did it slowly. That's why in "Kateřina pomalu zavřela oči." the new stuff is at the and in the same way it is in English.
If you chose "Kateřina closed her eyes slowly." than the main message is that she did so slowly, the word slowly is at the end and then you indeed do the same in Czech: "Kateřina zavřela oči pomalu.".
That all is valid for neutral intonation. If you pronounced the sentence "Kateřina SLOWLY closed her eyes.", than perhaps... But I can see why it is not currently accepted.