Replacing the gas cap on your Buick Skylark would not have caused the LPFET or EVAP test failure. Here is how the LPFET works... Your Buick Skylark has an EVAP hose which runs from the gas tank to the EVAP charcoal canister. The charcoal canister, usually located in one of the far corners of the engine compartment, is a container filled with charcoal pellets that absorb fuel vapors. The Low Pressure Fuel Evaporative Test (also known as the EVAP test) ensures that there are no leaks in-between the hose leading from the Gas Tank to Charcoal Canister... (and anywhere raw fuel exists).
During the smog test the smog technician must clamp the end of the hose leading to the canister and pressurize the gas tank with Nitrous Oxide via the EVAP test tool. The EVAP test tool will then calculate the drop in pressure due to any leaks. Technically there should be very little drop in pressure once the EVAP system (gas tank) is pressurized.
In order to fix your Buick Skylarks EVAP failure it will be necessary to locate the leak and repair the line. Depending on where the leak is located, this repair may be cheap or expensive, but the first step is to find where the leak is at. The gas cap is conducted on a seperate test machine which is attached to the smog check machine. The gas cap test is not part of the LPFET.