Translation:This shop is always busy.
In fact, I've never seen those kanji used for いつも, so it may go beyond usually.
Isn't "This store is always bustling." possible too? 賑やか = bustling; busy; lively 店=store, shop
"Crowded", as in there are a lot of people relative to the amount of space there is, would be 混んでる (こんでる). As Katsudamn mentioned, にぎやか has more of a connotation of being "lively" or "full of activity". For example, an elevator full of people would be described as 混んでる but not (necessarily) にぎやか.
In this exercise, the shop is being described as having a lot going on (にぎやか). It might also have a lot of people in it (混んでる) or not but the word choice of the sentence doesn't tell you either way.
I was just wondering if there was another translation of this sentence. Please let me know if you agree or not.
"This shop is always bustling."
According to the Wisdom English-Japanese dictionary にぎやか【賑やか】 busy, bustling
If I'm understanding you correctly, I guess it means that this shop always has a lot of customers.
The word is 賑やか, にぎやか, not ぎやか.
The に is not a particle in this case, but a part of the word 賑やか, which means lively, bustling, busy.
Please explain me. Why can't i say この店 instead このお店? What is a differance and when i should insert this prefix?
You will never hear anyone of the male gender say お店 and only some females use it, specifically women and girls who want to be extra polite or feminine.
It doesn't need to be but rarely is it not. Duolingo should accept it without.
Why does the tooltip for いつも say both 'always' and 'never'? Should that be reported?
No, that's correct; it just depends on whether the verb is positive or negative. "This shop is never busy" is essentially the same as "This shop is always not busy". (The tooltips are just hints after all ;) )