Elenco Digital/Analog Trainer XK-700K Output voltage off
The datasheet of the LM337 shows what the parts should be (the schematic is correct) and how to calculate the setting of the potentiometer and the input voltage to get an output voltage.
The 15VAC input has a peak voltage of 15V x 1.414= -21.2V and the rectifiers reduce it to about -20V. The 120 ohm resistor and 2k pot can be calculated for the LM337 to produce at least -18V.

Make sure that the input to the LM337 is at least -18V and check the values of the 120 ohms and 2k pot.