how am I supposed to write Russian translation with Latin alphabet, I tried all the possibilities I could think of (Zdes' net čaja) but non was accepted .... I am asking because there used to be an option to switch to Russian keyboard with duolingo but it is not possible for quite some time
I don't think Duolingo has ever provided a Russian virtual keyboard. You can just type your answers in Russian by adding a Russian keyboard to your system, whether the normal Russian layout or a phonetic one.
I don't know if anybody has ever fully understood the transliteration system, but I think it's safe to say it doesn't use "č," as it was targeted to those with only English characters available.
Check out my response to Christophe336542
English grammar references back up this up:
Of course, native speakers are not bound by such rules. But suspecting you're not one (A native speaker would have written "Why is 'There isn't tea here' wrong?"), probably best to follow the formalized rules.
A caveat here is that, judging from Psynderis's comment "There isn't any tea here" (which should be fine) isn't currently accepted. But there are technical glitches sometimes.