Translation:Czechia has neither a sea nor tall mountains.
I'm not sure how helpful this will be, but it usually sounds weird when only one of the nouns is "determined." A more common example might be "I have neither car nor bike" OR "I have neither A car nor A bike" VS. "I have neither A car nor bike." So I'd suggest adding "...neither sea nor tall mountains" to the acceptable translations.
In a statement like my car/bike example above, an article usually would be used: "I have neither (a/the) bike nor (a/the) car," as appropriate for the situation. Leaving an article out is more a matter of style, and there are probably situations where that stylistic change might not work too well.
Omitting the INdefinite article commonly happens with non-countable nouns, where using one would be strange. For example, “I have neither (the) sugar nor (the) flour” would be standard usage (based on the situation), and “I have neither A sugar nor A flour” doesn’t work.
But back to this sentence. Maybe it’s me, but using either article before “sea” sounds strange here. For one thing, when articles are used, they are usually paired, and here there is no pairing. For another, unless a sea can be in the middle of a country, I read the sentence as “Czechia has neither (access to a/the) sea nor tall mountains.” With that reading both articles feel out of place.
That said, and after checking the reports, I’d favor adding “neither sea nor high mountains” as an accepted alternative, unless it really doesn’t work from the Czech perspective.