Learn the refrigeration cycle. You are working on something interesting, a VRF system. What do all the valves and chambers in that box do? Figure it out. Learn the PE graphs, pressure enthalpy, it tells you everything you need to know about what the refrigerant is doing, temperature, density, at all points of the refrigeration cycle. If you get it down pat, you will know what should be happening at every point of the system, what temps and pressures, and then be able to figure out what is not right.
Learn motors. Why do they turn? How do they start? What do capacitors do? Motor theory. There are good books on this.
Learn flow controls. Tx valves, all the pressure control valves and their application.
As for skills, good piping practice. Learn to control your flame and temperature when welding; nothing like changing a tx valve, sweat, in a hole where you have 4" of room. It takes a couple of years of welding practice to be able to do it quickly and without problems.
HVAC is about keeping people or equipment comfortable. Product refrigeration is more complex. Meat, vegetables, frozen products, all have characteristics that need to be accommodated. There are great engineering books from manufacturers that detail this.
It takes a while to learn all this stuff. Another thing to remember is that when the freezer quits you have to fix it. Air conditioning only needs to work when there are people there. The exigencies of product refrigeration are quite different.