So, someone translated:
"Unable to accomplish anything"
That is indeed an excellent translation in that it conveys perfectly the meaning/essence of the original. It is not a word-for-word translation, of course, but a word-for-word translation often makes little sense between Japanese and English.
In case you wanted a literal translation, it would be something like "to be unable to (even) become anyone". IMHO, that would be far more ambiguous than "unable to accomplish anything". You may, of course, disagree.
「何者」, in this context with the negative 「なれない」, means "a worthwhile person", "someone who has achieved something", "a person worth mentioning", etc.
「も」 means "even".
「～～になれない」 means "to be unable to become ~~".
Thus, the phrase is saying that someone is (being) unable to become (even a bit of) a worthwhile person. If I am not mistaken, "to become a someone" even in English would mean "to become a worthwhile person", does it not? We are talking "becoming successful" and "not becoming successful" here.
Do you still feel that "unable to accomplish anything" is a strange translation? Quite a few users here seem to let the translated words get in their way of understanding the original. I know that for sure because I do answer quite a few questions here.
Translation is an art, not a natural science. The components used in one language do not have to physically correspond to those used in the other. If the essence of the phrase/sentence is maintained and the product (= translation) sounds natural in the target language, you basically have a good translation.