This question is usually asked when conversations about geothermal come up here, and for good reason because it’s a valid question. The long and short of it is that the amount of thermal energy that humans would be able to realistically extract pales in comparison to the amount already lost to heat flux through the surface, the amount generated via radioactive decay and frictional forces from gravity/tidal sources and convection in the core. Some answers on a relevant stack exchange answer with more general calculations [0].

Edit: Off the top of my head, it’s estimated that total core heat is approximately 10^31 J, heat loss at a rate of 47TW, (forgive me for not carrying out the calculation completely or for sources on those numbers, they can be easily searched, I’m on mobile), so the orders of magnitude difference already are so vast it’s a rounding error in siginificant figures.

[0] https://earthscience.stackexchange.com/questions/2302/can-th...