Formal:
* Uneducated
* Unlettered
* Nonliterate
* Inability to read and write
* Functionally illiterate (specifically refers to someone who can read and write but lacks the skills to function in society)
Informal:
* Uneducated
* Unschooled
* Ignorant (can be considered offensive)
* Illiterate (though it's considered somewhat blunt)
It's important to be sensitive when discussing literacy. Some people may find the word "illiterate" offensive, and it's generally better to use a more neutral term like "uneducated" or "unlettered."