Duolingo sometimes just randomly puts users to A/B tests so some people have access to immersion, and some don't. Also people from Europe often are forbidden to translate articles. Many people are disappointed because they can't use Duolingo's killer feature: immersion.
There exists many workarounds how to get translation functionality, but this is the simplest and most comfortable one.
Just install this extension, and the immersion tab will be unlocked for you! Of course it will only work if immersion is available for your current languages.