Translation:I climbed a mountain yesterday.
How nonchalant. "I climbed a mountain yesterday. Maybe tomorrow I'll swim across the Bering Strait."
"climbed up" should be an acceptable form of "climbed", especially when Duo gives you the word!
The audio wrongly puts the stress on the first syllable of 山. It should be yaMA, not YAma.
Expand the range of possible answers here and in all cases where the subject is not specified. Sentences like these depend highly on context which is not provided in these lessons.
Why not, "Yesterday, he climbed a mountain"? Or, "You climbed a mountain"? Or for that matter "it"? If you're going to drop the pronouns, don't make us guess the context. Just accept every English pronoun unless one is specified in the Japanese.
I'm sure any pronoun you entered would be added to the database eventually if you submitted an error report.
Whenever I hit this problem, I keep recycling the question and submit error reports for every pronoun. Still, it should just be written into the database for every question.