There are a lot of people who go "I tried to learn X through Duolingo and failed". Sure, that's probably true, because staring at the app is not how language learning works. Much like 100 years ago, people would have said you can't learn a language by reading a single book.

Duolingo is great for basics of the language, vocabulary and constant daily lessons. But you always need more. There's a whole language sphere out there. People actually using the language and whatnot.

I started studying French through Duolingo and about 6 months later I was like "I really need a grammar book and a dictionary, dammit". Year in, I was like "I should try reading news in French and maybe try a book."