Translation:It's not six o'clock now.
Learning words like this is not flawless. There are some words whose readings may change depending on context. For example, 何 (なに) can be read as 何 (なん), and 一 (いち) and 分 (ふん) together are read as 一分 (いっぷん), but clicking the kanji individually will not teach these alternate readings.
When you click on the Kanji sometimes It doesn't play the same sound, because Kanji makes multiple different sounds. So clicking everything individually may not always work. But sometimes In other lessons they provide a turtle mode. That is when learners learn to recognize a sentence character by character. But in this case, there is no turtle and it's not slow because it is teaching the learner to recognize a sentence as a whole. Like if they were in a conversation, there is no turtle mode on a person.
You're probably not wrong. There is no particular logic to when DuoLingo presents new grammar and vocabulary. It's basically random.
In this case, は marks 今 as the subject of the sentance. 今 means "now" or "right now".
So a literal translation of this sentance would be something like "As for right now, it is not 6 o'clock."
It sounds like "rokuji", because of the time marker 時 (ji). There is no pause between roku and ji when saying the time - it flows together like one word.
Listen for the first は and it will come right after it and before the では. Here is the romaji version to make it easier to pick-out what you are listening for in the audio.
"Ima wa rokuji dewa arimasen"
Why is there a "now" and a "今"? I find that they are completely useless in the sentence. If you were to say "it's not six o'clock" you would not have to add "now" after it because the person you are talking to understands that it is not six o'clock. Same goes with the Japanese sentence, the "今" doesn't do anything because the person you are talking to understands that it is not six o'clock