For all the time duolingo was teaching (jabłko = an apple) AND (jabłka = apples), and suddenly it is reversed. Is it wrong or I am not getting it correctly?
jabłka is Genitive form of jabłko - dotykać requires Gen form.
You can distinguish plural and singular from adjective - czerwonego is singular (plural would be czerwonych jabłek)
"Czerwonego" would be a bigger clue if we had already learned the genitive forms of adjectives. This seems to be Duolingo slipping in one of those "you don't know this yet, but ..." sentences. Depending on your learning style, some people find it "more interesting" when Duolingo slips those in, and some people find it maddeningly frustrating.