I just asked a native Polish speaker whether "Mam tylko jeden cukierek" is okay, and all she said was "No. Because przypadki have no mercy." But.... on the WSJP dictionary, it say it's both M2(masculine animate) and M3(masculine inanimate)?

In english, generally candy is thought of as american exoression. We have candy floss..the spun sugar ball you buy at fairs, otherwise it is a sweet or sweets.

So why is this genetive?

Unfortunately those with higher educations have to extol their knowledge on the ignorant masses. Though lacking a college degree I had used commonsense to start and operate a very successful business for 54 years, I'm sure if you log on to Google and Duda water Sports you will find remnants of it after I sold it at 78. There comes a time to call it quits and enjoy the fruits of your labors. Communication is the major factor in any enterprise and lack of it means almost certain failure. %4 years would indicate there was a modicum of truth in what I say. Incidentally I had 17 years in the printing trade offset and letterpress before starting my business and employing my associates while I worked at GE Aircraft the last 15 years as a chemical milling tech. I am very proud of the fact that during the 25 years of employment i garnered five substantial cash awards and two letters of commendation. I attribute this to working on my communication skills and presentations to staff. Communication on a ninth grade level for the sake of intelligent and understandable communication. Now I am proud to say using Doulingo I am expanding my communication skills while keeping my memory sharp and dementia at bay. all it requires is initiative. For this I thank Jellei and Duolingo, I may pass on but will not be in a vegetative state when I do. Thanks

In Australia a lolly is the same as a candy. I only have one lolly.

Come on! Is Polish not complicated enough without adding a group of random words which act like they are animate when they are actually inanimate?

What about candy corn? This is all over the place along with hard candy and sugarless candy.

Jeden kawolek cukierka"

I suggested: I have only got one candy. This ,to me, sounds more fluent in English and is probably how I would express it.

Mrs Mop is correct. In British English, it is a sweet. You can have one sweet. You would not have a piece of sweet (unlike the US English "piece of candy" described earlier).

Language is extremely fluid, the English I spoke in school seventy years ago is considered archaic in these times. Fifty years from now , we not be able to understand what is being said using today's terminology. We've gone from communication to dissecting to prove our grasp of it and in the process lost what Winston Churchill said abut communicating. Legalese is one of the biggest problems, say it in such a way that the unwashed and those sans a college degree don't understand. It's easier to take advantage of them that way. Common sense and a formal ninth grade education speaking.

Imagine how much more polish people could accomplish if their lanuage was more simple

Lawyers nowadays take pains to communicate simply. They lose business if they don't.