if the clutch is not engaging, it is impossible for you to get pressure on the low side and no pressure on the high side. What should happen is you have the same pressure on both sides. System should be ''balanced'' since the clutch does not engage. To be clear, you should for example be reading 30 PSI on the low side and 30 psi on the high side, because the compressor did not compress.

at 20 celcius, you should have something like 70-80 psi in your system (both low and high side) if your clutch does not engage. You are probably lower than that, if it is the case, your clutch does not engage because you are low on refrigerant, which means there is a leak.

I use a cheap gas with dye and a cheap uv flashlight to find leaks.