You could say המנגו מריח טוב = the mango smells good. but המנגו יש ריח טוב is not a correct sentence. The ל implies the ownership in the way the verb "to have" is constructed in Hebrew. Translated literally this sentence is "there is good smell to the mango". the ל incorporated the "to the".
Hebrew isn't as strict about word order as English. Both variants are acceptable.
That said, what is this sentence about? Mangos. The mango is the subject. So it makes sense to put the mango first, and the word that modifies the mango (יש) afterwards. But feel free to phrase it in any of these ways for the desired emphasis:
למנגו יש ריח טוב
יש למנגו ריח טוב
ריח טוב יש למנגו
But don't say the following. It makes you sound like Yoda:
ריח טוב למנגו יש
The other way around, I think: It's always /re-akh/, and /re-yach/ is an anomaly that's fading away. It's the pronunication of Israelies who immigrated from Poland and Russia up until the 30's, their most luminary example (for pronunication jokes) being Shimon Peres. Also you find it in songs from the 50's through mid-70's, because singers were taught to add it on the grounds that hyatus is not nice. https://youtu.be/oOw-NO7VzV4?t=49 (/ya yareyach/: יא ירח, addressing the moon).
As to your question, why something isn't accepted. Because it's impossible to include every single possible translation of every sentence. The translations are input manually into the system. So, whenever you write a correct answer and it gets rejected, you can report it and hopefully they will add it to the list of correct answers. You can try reporting this next time you come across this sentence.
Personally, I'd translate "pleasant" as נעים. To me, they are not exactly synonymous.
You need the article "a" in front of mango in English. Here's an article explaining it: https://www.quickanddirtytips.com/education/grammar/when-to-use-articles-before-nouns
That depends on whether you are using "mango" as a countable noun or as a mass noun. If mangoes smell good in general and that is the message you are trying to convey, then it's fine to treat it as a mass noun and forego the indefinite article. In fact, saying "a mango smells good" indicates that there is somewhere in the world a mango that smells good. That is not what you're trying to say.
The article you linked gives a similar example: Fruit is on sale.
Because native English speakers wouldn't usually describe the way something smells in that way. The most common way of saying it would be "The mango smells good." I think the translation "The mango has a good smell" is a more literal translation and maybe has been translated like this to help learners to better understand the way the Hebrew is constructed? "The mango has a good smell" sounds like something a learner of English would say.