Translation:She was surprised that you did not know about it.
The present tense "nevíte" might tempt you to translate into the English present tense ("... that you do not know about it.") And nowadays, many English speakers would accept that.
But traditionally English requires the past tense (here "did not know") after a verb of reporting in the past tense (here "was surprised").
That rule does not apply to the Czech, however.