Why does my air conditioning only work on passenger side?

Dual zone climate control? Does the temp go to hot on drivers side if you turn it up to the highest temp setting? That may help verify that the blend door is working. IF that is working , without knowing other Subaru 'quirks' , I would suggest having the system checked professionally to verify it has the proper charge of R-134.

Need to starr with an AC service first to ensure that the system is properly charged. We've seen several times whem the system is low on charge but partially blows cool on one side but not the other. Then the next step would be to diagnose any dual zone issues ( if your vehicle has dual zone HVAC controls)

Answer 1 (sounds like a blend door issue)