"Le" is the masculine definite article for "the". For euphonic reasons, it becomes "l' " before nouns starting with a vowel. (It sounds better to say "L'animal", and "Le animal" would not be correct.)
And where should you report mistakes?! Please don't tell people not to report mistakes here without telling them where to report them.
And here is a mistake. The translation "The animal's drinking milk" was rejected because of the use in English of the abbreviated form "animal's" instead of "animal is".
You should avoid such non-required contractions because the system cannot recognize them as correct. What the system interprets with "animal's" is a possessive case. Since this sentence does not have a possessive, it is considered as wrong.
No need to report.
Translation: "Computer says no!"
This is very poor pedagogical practice. There are two correct answers -- you reject one of them, although the error messages themselves indicate that your system is perfectly capable of understanding the usage and the distinction from the possessive form.
In many tens of cases you allow multiple answers where the source language expression has multiple target translations. And, you routinely allow "I'm" and "I am" as synonyms, to take one case in point. "I'm" is not a "required" contraction. "I am a robot" is always grammatically correct, sufficient if you like.
Far from not needing to report this, it's pretty important that you stop miseducating people about correct English. See the following more extensive discussion.
Please: don't be so dismissive. You are capable of introducing a false meme into the minds of millions of people -- literally millions. Look at the (rather impatient) way you have had to laboriously explain the complexities of aimer vs adorer to all those people whose teachers have half-educated them around the world.
I am not sure you are addressing the right person here. I do not belong to Duolingo's staff, I am a volunteer who has tried to help people for over 3 years to complement the original 'teaching per example' method.
All Tips and Notes in the course have been added by volunteers; answers to questions on the forums are provided by volunteers; grammar and syntax explanations and information about the limits of the system are given, again and again by volunteers who have no control on algorithms Duolingo has put in place; all reports to Duolingo's staff and developers are documented and sent by volunteers; volunteers read all user reports, one by one, and send users emails when their suggestions are accepted; volunteers add variants and create new sentences in the incubator. But volunteers do not control the system and you cannot expect them to add "Noun's Verb-ing" as correct in every single sentence where a 3rd person singular continuous present is possible.
The discussion you pointed to has not been commented by any Moderator or course contributor. Yet, Burner0 gave interesting insight on the complexity of planning for every variant.
There is nothing like "computer says no". But there are technical limitations, and not everything can be automated. It is easy for the computer to recognize "I'm" for "I am" for it can be nothing else. But the system has not been programmed to distinguish a possessive case from a noun followed by the contraction of "is" to "'s". Similarly, the system does not accept "it's" because it can be mistaken for "its".
But the system also produces gross mistakes. For instnce, it gives as correct answers such forms as "he's a car" instead of "he has a car", because the algorithm provides that " 's" is correct to replace "has" anytime and anywhere. In such cases, in spite of our repeated reports and warnings, Duolingo has not cracked that tough nut.