It's a global scandal that so many adults are unable to read and write: Something like750 million adults are illiterate, according to official statistics. It's probably double that if you count anyone who can't meaningfully read and write to function in their society.
For adults who miss out on school, there are very few opportunities to get that second chance. And school is not really designed for adults. There should be special provisions for adult literacy, with as its starting point, respect for what they've gained throughout their lives. They can't be treated as stupid; the only thing they don't know is how to read and write. It's shocking that people don't invest in that.