Speaking Japanese, in depth
Counting beats, not syllables
Japanese is not measured in syllables, as English is, but in moras: units of time that all last the same. Each ordinary kana is one mora, and so are ん, the small っ and the second half of a long vowel. That is why 日本 (にほん) has three beats and 東京 (とうきょう), four, although in English we would count two syllables in each. Combined syllables like きょ take up just one, because the small ょ does not count separately. A good exercise at the start is to clap once per mora.
Respecting that rhythm is not a detail of accent: it changes words. ビル is «building» and ビール, «beer»; おばさん is «aunt» and おばあさん, «grandmother»; きて is «come» and きって, «stamp». English speakers find it hardest to hold the long vowel and the pause of the っ, because in English length alone does not tell words apart and the ear lets it slip by. The solution is to exaggerate them when practising, until the right length comes out without thinking, and to listen to pairs like these until you hear the difference.
The vowels you can barely hear
In standard Japanese, i and u become voiceless when they fall between voiceless consonants, or at the end of a sentence after one of them: they still take up their beat, but they can hardly be heard. です sounds like «des» and ます, «mas»; すき (liked) sounds like «ski» and した (below), «shta». Nobody writes it that way, but that is how it is pronounced, and anyone who says «de-su» with the full u sounds like a textbook. It is one of the things that brings pronunciation closest to a native speaker's.
It is best not to overdo it, because it is not a fixed rule: it varies with region, speed and speaker, and in slow or emphatic speech the vowel comes back. The way to learn it is not to memorise where each vowel drops out, but to imitate what you hear. That is why it is worth listening to phrases before reading them in romaji, which is exactly what tempts you to pronounce every letter, and paying attention to the rhythm at normal speed more than at slow speed.
The accent is a matter of pitch
Japanese does not mark a syllable with more force, as English does, but with the height of the voice: each word has a pattern of rises and falls. In Tokyo Japanese, 箸 (はし, chopsticks) starts high and drops, while 橋 (はし, bridge) starts low and rises, and the difference also shows in the particle that follows. 雨 (あめ, rain) drops after the first mora. There are pairs that can only be told apart this way, although in practice context almost always clears up the doubt.
For beginners, the advice is neither to obsess over it nor to ignore it. You do not need to study the pattern of every word to be understood, but saying everything in a flat tone, or with the stress accent of English, is what most gives away a foreign accent. What works is imitating whole phrases, with their melody, rather than single words. And it is worth knowing that the app's voice is the device's: its intonation is usually good, but not always perfect.
Getting the most out of the microphone
Speech recognition turns what you say into text, and the app compares it with the phrase. For it to work well: a quiet place, the microphone close by and the phrase said whole, at normal speed and without separating the syllables, because the recogniser is made for natural speech and not for syllable-by-syllable dictation. If it answers «try again» several times in a row, it is worth hearing the phrase again before repeating it. And the browser needs permission to use the microphone, which is usually given just once.
It is worth knowing what it measures and what it does not. The recogniser looks for the most likely words for what it heard, and sometimes corrects an imperfect pronunciation on its own, so «perfect» means you were understood, not that your accent is flawless. Conversely, a noise or an uncommon word can produce an unfair «almost». It is a tool for losing the fear of saying the phrase and checking that it is understood, not a pronunciation exam. And the recognition is done by the browser, not by Hanasuka.