Estimate function expression for TC and MC
As the OP corrected in the comments, using $K=20, w=800$, and $r=500$, so $TC = 10000 + 800L$, we get , in the $\{Q,C\}$ space


This looks pretty text-book like. The fact that there are levels of labor for which output is reduced, tells us that for these levels we have to employ more of the "fixed" factor, capital (which thus stops being fixed).
The function as is, with capital fixed at $10000$, stops having economic sense after $L=9$.