"Murung tidak akan menyelesaikan apapun."
Translation:Feeling down will not solve anything.
The Indonesian sentence listed (Murung tidak akan menyelesaikan apapun) feels to me like an odd way to say this. Does saying it that way sound natural to Indonesians? I might have added "Rasa" in front of "murung," but, okay, whatever.
I translated it as "Gloom will not finish anything," which does sound overly stilted (if literal) to me, but I couldn't think of a good idiomatic equivalent in English.
You made a point. "Murung" is an adjective, so it cannot be a subject without being proceeded by a gerund.
I had the similar question in the previous lesson. "Indah itu baik." / "Beautiful is good." According to a native speaker, this sentence is grammatically incorrect. For more details, please refer to HiNative.
If the same logic can be applied, "Menjadi murung tidak..." is the complete and grammatically correct sentence.