Here's why:
* Prefix: A prefix is a word part added to the beginning of a word to change its meaning. "Posture" doesn't have any word parts added to the beginning.
* Suffix: A suffix is a word part added to the end of a word to change its meaning or grammatical function. "Posture" doesn't have any word parts added to the end.
It's possible you might be thinking of related words that *do* have prefixes or suffixes, such as:
* Postural: This word has the suffix "-al" which means "relating to".
* Posturing: This word has the suffix "-ing" which indicates a continuous action.
Let me know if you'd like to explore any of these related words!