The phrase "having a knack" implies a natural or acquired talent for something. Antonyms for this phrase would be "lacking skill," "inept," "unskilled," "incompetent," or "clumsy." These words suggest the opposite of someone who has a knack for something, and instead indicate someone who may struggle with the task at hand. Synonyms for "having a knack" could include "adept," "talented," "proficient," "skillful," and "gifted." These words describe individuals who possess a natural talent or have developed a skill through practice, experience, or education. Knowing these antonyms and synonyms can help one to better understand the nuances of language and how words can be used to convey different meanings.