"It is tiring to run a lot."
Indeed, I would suggest that "Running is very tiring" would be a closer translation, wouldn't it?
Somebody more fluent than me is free to correct me if I'm wrong, but I think a more literal translation of Duo's Japanese here (「たくさん走るのは疲れます。」) would be "As for running a lot, I get tired." This is because つかれます actually functions as a verb in the given sentence, and its subject should be inferred as "I" by default. (Remember, は is not a "subject" particle, and "Running a lot gets tired." doesn't make sense.)
I think a Japanese sentence closer to "It is tiring to run a lot." would be「たくさん走るのは疲れることです。」(literally: "As for running a lot, it's a tiring thing." or "Running a lot is a tiring thing.").
As I understand, there is no default subject. Duolingo tends to put "I" because a subject is needed in English. Other subjects are usually also accepted. While I am not certain the construction is appropriate (I would need to ask my sister, who has lived in Japan longer than most Duolingo users have been alive), I might think of this as "As for running a lot, it gets tired."
What is the difference with or without のは in this sentence? たくさん 走るは疲れですand たくさん走るのは疲れです?
You need to make a nominal group for the は particle, hence the use of の. I've read somewhere that it used to not be the case in old Japanese, but I don't know much about that. In any case it is safe to assume your first sentence will sound wrong or at least to a native speaker (I just asked a japanese friend to confirm).
Also, I guess it's a typo but 疲れます,not 疲れです
たくさん走るのは疲れます。Is my kanji not correct? It didn't accept it. Grammar is same as their correct answer. Just has kana instead of kanji.
Why does たくさんCome before はしら? Does frequency always come before nominalize verbs?
The subject of the Japanese sentence is たくさんはしる(の), or "running a lot". For たくさん to modify はしる in this sentence, it has to be before it. (The の after the verb makes it a noun phrase instead of a verb phrase.)