Hello and welcome .I have no Idea but following the thick pipe from the compressor until you find it is the usual method (which you have obviously tried) It could be on the compressor (rare) or behind the firewall before the thick pipe gets to the expansion valve .
It has to be between the compressor and the expansion valve. Check out Renault Kangoo (similar thing) you might have more luck there.
When you find it please come back and let us know.
For what it's worth to actually get to the (clearly visible) LP valve on my wife's Zafira I have to remove the air filter intake pipe along with the MAF sensor. They tucked it away back there for some reason.