Translation:The small, red razor is in the glass.
I think you know by now (and what to do about it). "The little red razor" is fine in English.
The problem was and is the word little. It happened to me many times in different sentences, that little was not accepted and I was forced to write small. But I never know, when little and when small is required. Is there a general rule for it? Whenever I would use the German word klein, kicsit, I write in English little and for eng/szük - small. But it doesn't work at all.
"eng" is better in English as "narrow" (e.g. a space between two buildings) or "tight" (e.g. clothes). "small" does not mean "schmal, eng".
"klein" is "small" or "little" - nearly always, they mean the same thing.
The most prominent exceptions are "little" meaning "wenig" (He earned too little to survive) and "a little" meaning "ein wenig" (I had a little beer this evening = I had some beer this evening), which cannot be replaced by "(a) small".
But most of the time, whenever "little" is accepted, so should "small" and vice versa, and similarly for "big" and "large".
Unfortunately, as we know, on Duolingo the people who write a sentence might include only one of the synonyms and not the other one. (Similarly with "next to / beside", "below / under / underneath", "over / above", etc. etc.)