The problem is that many people see Duolingo (and Duolingo partially advertises itself this way) as the first and last step needed for learning a language. On the contrary, Duolingo is a good first step, but it's only a first step. Instead, simultaneous with Duolingo, you should be using a spaced repetition system to study vocabulary, like Mnemosyne, Anki, or Memrise. After you get significantly through the Duolingo tree (or finish it, for a smaller tree), you need to start immersing yourself in the foreign language - consuming media in the language and talking with others who speak your target language, at the very least. Another good tool after Duolingo is Clozemaster, since it helps you get exposed to lots of vocabulary and sentence structures.

It depends on your language, but widely spoken languages have "easy language" media available, which is a good place to start. But also, look on YouTube for native speakers talking, listen to podcasts, read books (keep Wiktionary or WordReference at hand), and find people online! It'll be hard at first, but you'll quickly get a lot better.

In addition, Duolingo (and every other language learning tool) is significantly better for some languages than others. There is now the "story" feature for some languages, which helps a lot with reading comprehension, especially after you get past the first few easy tiers of stories.

I'd say that completing the Duolingo tree in German got me through a similar level of grammar that my four years in high school did, but what it is lacking is the many hours of listening practice and vocabulary practice.

Disclaimer: I have no connection to Duolingo or any of the other tools I mentioned; I am simply a fan.

Credit where it’s due though. The low barrier to entry and habit-forming nature got me learning Italian. I’m definitely not fluent but I can have conversations with people in cafes and restaurants and it’s pretty fun! Recently I’ve moved on to Babel and I like that too.