However, you could consider using prefixes that relate to the meaning of legible, which is "easy to read":
* Il-: This prefix means "not" and could create "illegible," meaning difficult to read.
* Un-: This prefix also means "not" and can be used similarly to "il-".
* In-: This prefix means "not" but is often used with words of Latin origin. It could be used to create "inlegible," though it's not a common word.
It's important to note that these prefixes might not create words that are commonly used in the English language.