Why I can't use "nice" instead of "pretty"?
You can. If your answer was rejected, you have to report the complete sentence.
I keep trying to translate hezký as "beautiful", which in English is the same as "pretty", only more standard and formal, and more common for inanimate nouns. But the course seems to only accept "pretty". Is there a reason for this?
Yes. Hezký is not that 'strong' word. Use krásný for 'beautiful.'
Thanks! Very helpful.