I see you want to make a current mirror. First things first.
To simulate this circuit, and to learn how the Zener works, you need a "complete" circuit. Don't leave the collector of Q6 or Q7 disconnected. Use a resistor of 40 ohms, then connected to GND. Similarly,on the collector of Q7, connect 40 ohms to GND=0.0 Vdc.
Just for a moment, keep your -5V negative rail, but choose a 4V Zener. You need to make sure the Zener "turns on". Keep your Zener = 5.1 volts, but for the simulation , use -8.1V rail voltage.
Suppose the Zener datasheet says the Zener needs 3mA to operate nominally; then the R18=10K resistor has to be 1K.
Now you are ready for the overview. The voltage from 0.0Vdc (across R18) to the -8.1 rail is 8.1 volts. 5.1 volts are across the Zener. The voltage at the node of R18, Q6base and Q7base is -3.0 volts. The voltage at the Q7 emitter is -3.7V, voltage at the Q6 emitter is -3.7V. The current thru R9 is (8.1 - 3.7) / 100 = 44mA, and Q7 base current is 44mA/100 = 0.44mA. Q7 has to operate linearly. So the voltage across Q7collEmmiter = 2V. Voltage at Q7 collector is -1.7V. The resistor from Q7 collector to 0.0V has to be 1.7/44mA = 38.6 ohms = 39 ohms, that is why I chose 40ohms when I started the discussion.