Differential equations: Why does DSolve return the same DSolve expression?

I am trying to find the closed solution for the classic chemostat model with a species and a resource (Monod response) using the DSolve () function. However, after running for 10 minutes, I recover the same equation.

In:
DSolve[{ x'
R & # 39;
{X

10 minutes later

Outside:
DSolve[{Derivative[1][x]
Derivative[1][R]
{X