"She was surprised that you did not know about it."

[Praha2017]

"Byla překvapená, že jsi o tom nevěděl" - why is this translation wrong? And how would you otherwise translate this sentence?

[VladaFu]

We would translate it exactly as shown above. Czech does not use English tense agreement in reported speech. We do not shift the reported speech into past when it was originally said in present.

Or do you mean how would we translate your sentence into English? "She was surprised that you had not known about it."

[Praha2017]

That's what I meant, thanks.