Both "en" and "dans" mean "in" but "en" is never followed by an article, and it is used in certain fixed expressions, like this one, en lieu sûr, in a safe place. Often "dans" refers to more physical "inside-ness" and "en" is more abstract. For example: "dans la classe" means "in the classroom" while "en classe" means "in class."
when you say en is never followed by an article, what about:
Nous croyons en la démocratie. - We believe in democracy.
je crois en l’amitié - I believe in friendship
These are example sentences from duolingo - I think the definitive article is needed because it means believing in the concept of democracy and friendship. Therefore the French sentence needs to express a generality.
It's wrong because "en" is never followed by an article
I am answering as a native English speaker, and not a grammer expert. In English, after the word "in", you usually use an article before the noun, and if there is an adjective modifying the noun, I think you always need an article. You don't use an article when the noun is used to represent an idea rather than a specific spot "I am in bed" means, more or less, "I am ready to sleep (or something one might do in bed)." "I am in school" means about the same as "I am a student." "I am in a bed" would suggest 'I am not sleeping on the floor, etc." "I am in the school" would indicate a particular school. I could go on, of course. It is amazing how much a very little word can so change the meaning of a sentence.
I'm not sure I agree. if I say I'm in bed, I mean I'm in bed, mine or, if I've got lucky, comeone else's. What I thought interesting about this sentence was that I wrote 'you are in the right place' and I got a note saying I'd used the instead of a. Which led me to think we would almost never say 'you are in a right place' or 'you are in the safe place'. A little word, as you say, but very particular.