Many people built the project and it worked fine except it has a square-wave output, it does not have voltage regulation and it does not turn itself off when the battery charge becomes low.
1) Use a high capacity battery.
2) Measure the outputs of the CD4047 to see that there is a 50Hz or 60Hz square-wave with a peak voltage that is the supply voltage.
3) Measure the outputs of the LM358 opamps to see the square-wave with a peak of a few volts.
4) Measure the emitters of the 2SC1061 transistors and the transistors Q3 and Q4 to see the square-wave but with about 2.5V and 1.5V peak.
5) Measure the collectors of the output transistors to see the square-wave that is about 23V p-p.
Your schematic is old. A couple of changes make the circuit more reliable. See my modified schematic.