What literary device is used in this sentence and why: "a man's errors are his portals of discovery."

The literary device used in "A man's errors are his portals of discovery" is idioms.

Idiom is a phrase with figurative and literal meaning. This is a formulaic language, a fixed term with no literal meaning and contains pause fillers (um, uh, etc.) sometimes. The mobility of the idiom is based on the phrase where some idioms can changed according to the situation while some remains fixed.

The given idiom is a fixed idiom. The meaning of the given idioms is when a person’s mistakes are analyzed, then the level of the mistake tells about the person’s knowledge. If a person does blunders then the person does not know much about certain thing. If the same person does trivial mistakes then it is known that the person is becoming well-known in particular field or quality.

For example, Einstein, George Gregor Mendel and many scientist have discovered many big things after learning from their blunders. So, the mistakes are used as pathway for their discovery.