"Let upon" is not a recognized phrasal verb or expression. Therefore, it doesn't have any antonyms as such. It's possible that the intention was to use "let down," which means to disappoint or fail to meet expectations. In this case, some antonyms might be to "succeed," "impress," or "fulfill." Alternatively, if "let upon" was meant to express giving permission or allowing something, some antonyms could include "prohibit," "forbid," or "deny." However, it's important to clarify the intended meaning of the phrase before discussing antonyms or synonyms.