Why cannot we fill a cylinder with a gas?
Refilling of cylinders with gas (I think u are asking for LPG refilling) is not easy job and it is dangerous to do at home by ourselves. A strict procedure should be followed in a restricted place. It must have a "recent" hydrostatic test date, or in cannot be filled per procedures and codes. It must be filled at a "legit" filling station (with proper equipment) by someone who is trained and knowledgeable in the transfer of compressed gas.
Calculations are made as to how much gas is in a cylinder, and they involve the type of gas, the temperature of the gas, the pressure of the gas, and the volume into which it is compressed. There are some other variables that must be taken into consideration when filling a tank with compressed gas. Fatalities occur now and then that are associated with the failure of a tank.