Technically, 54% of US adults read at Level 2 or lower as measured by PIAAC. (While grade equivalence claims are common, it’s not backed by any data, so many of us are abandoning that phrasing … plus what “6th grade” means is vague and cultural).
A better way to understand this is that a reader at Level 2 has trouble making meaning across multiple texts, especially if those texts contradict each other. Level 1 means they have difficulty accessing information via text.