Translation:She is sitting in the forest and is playing the flute.
"she is sitting in the forest and she is playing the flute" What's wrong?
There's no need to be that precise with the translations. It should be accepted, I guess it's just not in the database yet.
Did you try leaving it out and get an error message?
There are accepted alternatives without a second "is", so perhaps you made some other mistake.
What was the entire sentence that you typed?
Are musical instruments a rare case where German doesn't use the definite article but English does?
I suppose so.
Another exception is "services", where English might say "I read the newspaper every morning and I listen to the radio every night", with definite article for "consuming a service", rather than referring to a specific newspaper or a specific radio box; German could say Ich lese jeden Morgen die Zeitung but also Ich lese jeden Morgen Zeitung and I would only say Ich höre jeden Abend Radio, not Ich höre jeden Abend das Radio.
The word "playing" is not in the word bank so cannot complete the correct sentence.
What is wrong with She is sitting in the forest and plays the flute. ?
You're mixing tenses -- present continuous "is sitting" with present simple "plays".
Either "is sitting ... and (is) playing ..." or "sits ... and plays".