the mechanic installing the lineset and condensing and evaporating units needs to pull a vacuum from a vacuum pump or at least an old fridge compressor to make sure there is absolutely nothing to compromise the refrigerant.
Once the vacuum is applied - the gas block valves can be opened and the system checked for run pressure on both liquid and vapour side. If your AC has a TXV on the inner air handler the superheat is already managed, you need to set the subcooling.
subcooling means the difference of the actual liquid line temperature and the saturation temperature according to the refrigerant being used. e.g. liquid line pressure after condenser at 278 psi means that the saturation temperature is 51C on the liquid line, We now measure the actual temperature of the liquid line with a thermocouple. We should be seeing about 47 C - that shows that the condenser removed 6C heat from the refrigerant.
AC installation does not simply mean - laga ke thok do - gess daalo - paisay pakro aur faraar hojao.
Please bear in mind - if the refrigerant has been exposed to an install like that in the past and then it was pumped down again to enable removal of the unit - the refrigerant is adulterated. You can somewhat verify this from measuring the run pressure vs temperature. If that is the case, you need to evacuate the charge and fill fresh refrigerant in there after vacuuming it out.
I did fix such issues in Karachi in my aunt's house - its been a long time now but that AC still performs like a champ.