knowing a theory without being able to apply it
Idiomatic expression referring to someone who has book knowledge but lacks practical understanding or ability to apply it, originally from the Analects of Confucius.
He majored in economics, but he was all theory and no practice, and failed at actual investing.
From the Analects of Confucius (論語), literally 'one who reads the Analects but does not understand the Analects', used idiomatically to describe someone who knows theory but cannot apply it.