An error in a fill-in-the-blanks test?

Question

https://progress.lawlessfrench.com/my-languages/french/tests/take/2122084

Valentine et moi prenons aussi des cours de rockle vendredi. Elle est une très bonne danseuse, et quant à moi, je fais de mon mieux !

This was marked as wrong, for the "Elle est" part. It says that I should use "C'est une très bonne danseuse" and points me to this lesson. Après moi, y a quelque chose qui cloche là... Is it really wrong to use "Elle est"? If so, why?

I understand that, in general, you'd use "c'est" when it's followed by an indefinite article, but that rule doesn't seem to apply in this case. Correct or not, it just doesn't sound right to me. Moreover, the fact remains whether "elle est" is actually wrong here.

PS: btw, the option "Send email notifications of new answers" here below doesn't seem to be working. I've seen people answer my questions and I never got an email for that.

Accepted Answer

Hi Diego,
No, it is correct, you cannot say in French -
elle/il est un/e .... it has to be -
c'est un/e ...
This is often queried and difficult to justify it is just the way it is.
You can say:
Elle est bonne nageuse or c'est une bonne nageuse 
C'est mon frère ( not  - il est mon frère)
etc.
Regarding your other issue, I am sending it to someone in tech admin to deal with it. They will probably contact you separately by email.