Praise ignorance, for what man has not encountered, he has not destroyed.
So far I have:
ευλογει αγνωσιαν, ὅ γαρ ανθρωπος ουχ ευρηκεν, ὅ ουκ απολωλεν.
I am struggling with the relative pronouns. Is it OK to use a relative twice (as I did) without a referent when its generic? Please excuse the lack of accents. 
You don’t need a second relative, just the first one, and the lack of referent seems correct to me.
ἀπόλωλα, despite its active endings, has a passive sense – you need ἀπολώλεκεν.
A few small suggestions:
ἐπαινέω is a better word for ‘praise.’ This is what is used in Plato’s dialogues in a similar context (to name one example).
Abstract nouns often take the article; hence, τὴν ἀγνωσίαν. Likewise nouns referring to an entire class: ὁ ἄνθρωπος, ‘man,’ i.e. the human race.
I have a sense that the word order of the last part could be polished up a little bit. Putting it all together, I would say:
ἐπαίνει τὴν ἀγνωσίαν· ὃ γὰρ οὐχ εὕρηκεν ὁ ἄνθρωπος οὐκ ἀπολώλεκεν.
Or, if you don’t want it to be a command to a single individual, but a general statement, “One should praise…” you could use one of the verbal adjective in -τέον (Smyth 2149-52):
ἐπαινητέον τὴν ἀγνωσίαν…
Thanks for the feedback. 