Single supply opamp with AC coupling
Because there is no load on the capacitor, there is no current to charge the capacitor, and there will be 0V over the capacitor. If the op-amp output has a DC bias of 6V, so will the other side of the capacitor as well. When you put the resistor from capacitor output to ground, it sets the ouput DC bias and lets current flow via capacitor to charge it so there will be 6V over the cap.