There also might have been some other typo. Or Duo does not have its day. That said though. In a short sentence like this when you are saying about yourself that you are this or that (katerina, teacher) you would actually use the personal pronoun in most cases. Not sure if it is because of the use of TO BE or because such sentences typically begin the conversation but to say just "jsem katerina" or "jsem ucitelka" is very unusual. Not wrong, just not typical.
No, the subject doesn't have to be expressed, even if it's still there grammatically/formally. A lot of languages do the same, they are called pro-drop languages ("pro" stands for "pronoun"): https://en.wikipedia.org/wiki/Pro-drop_language
- CZ: Já jsem Kateřina. = Jsem Kateřina.
- ES: Yo soy Catalina. = Soy Catalina.
- FI: Minä olen Katariina. = Olen Katariina.
- PL: Ja jestem Katarzyna. = Jestem Katarzyna.
- HU: Én Katalin vagyok. = Katalin vagyok.