General synonyms:
* Inability to read or write
* Lack of literacy
* Functional illiteracy (for those who have basic literacy skills but struggle to understand complex texts or apply their reading skills in real-world situations)
* Non-literacy
* Illiteracy rate (refers to the percentage of people in a population who are illiterate)
More specific synonyms:
* Analphabetism (formal term for the inability to read or write)
* Ignorance (often used in a broader sense to refer to lack of knowledge or understanding)
* Uneducated
* Unschooled
* Unlettered (implies a lack of formal education)
* Inability to read
* Inability to write
The best synonym to use will depend on the context of your writing.