Like many words in English and Spanish, "escalera" can mean several different (but related things). It can mean stairs, staircase, stairway, ladder, or a run or sequence of things (like a straight in poker). It is sometimes pluralized to mean stairs, but not always. It would be nice if Duo would have a sentence with a little more context like: "Necesito una escalera para subir a mi techo" (I need a ladder to get up on my roof). But this would be a bit more complicated than the sentence that they have given us. I guess they are trying to keep it simple. So, I think they should accept any of these meanings.
When you focus directly on English you venture off course. The most important thing is understanding what a Spanish statement MEANS, not what it says. This means, leaving the refinements of English out of it. Like don't think of "rojo" as meaning, "red." Instead, think of "rojo" as being the color which certain apples are, the color of Santa's suit, and of blood. Think of "escalera" as a series of steps. This is its basic meaning. Think of it that way because that is what it is. Then to translate to English use the color of blood and such. And use whatever English word a series of steps might be called.
When I made the comment I was just offering an explanation for why Duo might not accept the answer, not making some judgement on whether it should/shouldn't be accepted.
For what it's worth I'm inclined to agree with you, but I think in these situations it's best to just use the report button and hope the people in charge agree too.
Stairs, staircase, stairway, and ladder are all correct translations.
I understand where you're coming from. But if you were my Spanish neighbor and you asked to borrow my escalera (you didn't know the English word 'ladder'), I would immediately know you were needing a ladder; i.e., I would never think you wanted me to disassemble my stairs and give them to you ;). I guess it would be the same if the situation were reversed.
According to wordreference, escalera can mean stairs or staircase: No funcionaba el ascensor y usamos la escalera. = The lift/elevator was out of order so I had to take the stairs.
Stairs can also be translated as plural escaleras.
In some cases, what is plural in one language can be singular in another language. In this case, stairs are plural in English, but can be either in Spanish. If the subject is plural, then so are the verb and adjective.
La escalera es roja.
Las escaleras son rojas.