* If you mean the ability to identify synonyms effectively, then the antonym would be something like lack of vocabulary, poor word choice, limited understanding of language, or inability to find equivalent words.
* If you mean the ability to use synonyms creatively and effectively, then the antonym might be lack of creativity, repetitive language, unoriginal writing, or lack of rhetorical skill.
Essentially, the antonym would be the opposite of being able to identify and use synonyms effectively.