https://en.wikipedia.org/wiki/Commodity_fetishism

In Marxist philosophy, the term commodity fetishism describes the relationships of production and exchange as social relationships among things (money and merchandise) and not as relationships among people. As a form of reification, commodity fetishism presents value as inherent to the commodities, and not arising from the interpersonal relations that produced the commodity

DR; TL; It's when you don't recognize the Labor Theory of Value.