The computer commands that a vacuum be pulled in the fuel tank, then measures the negative pressure in the tank to be sure that the vacuum is present, is correct, and that it holds. In the case of a large leak, it cannot pull a vacuum so the PCM sets the P0455 code. If the leak is smaller, it sets the P0442 code. Newer vehicles have an even tighter test for smaller leaks (P0456). It also means that the test has failed on two consecutive test cycles.
The fuel cap is the only thing that a typical owner can access. Check to make certain it's tight and that its seals are clean and intact.
If this was the result of a state-mandated emissions check, they should have also tested your gas cap. If not, you can try swapping it.
If you determine that the problem is not the gas cap, your best recommendation to locate the repair is to hire a shop (probably the dealer) that has a smoke generator to locate the leak. Likely fault locations are the CV solenoid, canister, fuel tank filler grommet, one of the seals on the top of the tank, one of the many hoses in the system, the purge valve, or an electrical fault with the pressure sensor or one of the solenoids.