Single Supply Op - Amp circuit not working
An op-amp amplifies the difference between its inputs, not the difference between an input and whatever happens to have the ground symbol attached to it in your schematic. Look at the dual-supply version of the inverting amplifier:



It amplifies voltage differences from ground because the non-inverting input (+) is attached to ground. It could just as easily be any other voltage, and this is why single-supply op-amp circuits can work.

In your circuit, the non-inverting input isn't attached to ground. It's attached to 6V. An op-amp with negative feedback will strive to make its inputs equal through the feedback path. Your input is -2.5V, and it wants the inputs to be equal, so the voltage drop over R4 should be \$6V - (-2.5V) = 8.5V\$. The voltage drop over R3 should be equal, since they are the same value, and the op-amp bias current is assumed to be none. Thus, the output would have to be \$ 6V + 8.5V = 14.5V\$. Of course, this is impossible for an op-amp with a 12V supply, so it does the best it can manage, 11.01V.

You can decrease the voltage at the non-inverting output by making R1 bigger and R2 smaller to move the virtual ground to a lower voltage, but you won't be able to move it all the way to 0V relative to ground because an op-amp generally can't operate all the way to the rails. Read more about common-mode input range to learn more. You could use a rail-to-rail op-amp to get closer. Or, you could continue to use a negative supply. Or, you could add a voltage to your input with a zener diode in series with it. Or, you may be able to AC couple the inputs. Which option is best depends on what the desired behavior when the input isn't -2.5V.

Here are some ideas for converting -2.5V to a positive voltage, without an op-amp: