Translation:Czechia has neither the sea nor tall mountains.
"Czechia has neither the sea nor tall mountains" is incorrect, or at least very unnatural.
I'm not sure how helpful this will be, but it 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.
Czechia doesn't have A sea or tall mountains. The definite article in the provided default translation suggests that you are talking about a specific sea, like the Adriatic, for example. The Czech sentence means that there are no seas at all in Czechia, doesn't it?
The translation 'high mountains' rather than 'tall' was marked wrong. In UK english this seems to be more natural. I have used the report button.