Translation:This is not okay.
@Sabina - I agree. But the issue is that there are hundreds; possibly thousands of different ways to say the same thing. Duo developers probably pick only the few most common variants or just a single dominant one. There are thousands of questions here, and having hundreds of responses programmed for every single question is too daunting a task. And there might still be the 128th variant that will be missed, and would result in raised eyebrows :-). All this is not going to help us learn Chinese any better anyway. My 2 cents.
I'm a native Chinese speaker learning German & French, just out of curiosity I tried this learning Chinese section. As had always happened in many languages, one simple sentence mean different things in different scenarios.
Here, "这", which is, "this", might be just anything, and this means that "这个不行" means a lot more than "this one is not OK".
"这个不行" might mean: 1. "这样做不行(建议你别这么做)", you can't do this ("you'd better not do this"). 2. "这个东西不行(这东西不适合这里)", it is not proper to put this thing here. 3. "这个方法不行", this method does not work.
& more as more scenarios coming in.
Dictionary definition：Satisfactory, all right, okay . So it it used correctly.
Why not just 不行?
Why do you need 这 when you've already conceded there is no object (or rather subject in the given context)? "This" would denote a subject which will need a counter in most Asian languages, particularly Chinese; and especially at this stage where we are learning the grammar and not being casual to omit bits and pieces.
For example, beginners who are learning English need to be taught "Do you want something to drink?" instead of the casual "Drink?", which will just confuse them; not knowing whether "Drink" is a noun or verb; what is the subject and predicate of the sentence; and whether the construct is a sentence, phrase or just erroneous grammar.
That's what I think, not sure about others.