Have a Nissan 98 Maxima which shows an engine error code of P1446, which indicates a closed evap valve sensor. How do I fix it?

Evap diagnosis can be difficult, especially on this system.
Hands-on diagnosis is crucial to pin-point failures. I'll try to help you:

This solenoid valve is used only for diagnosis, and usually remains opened.
This solenoid (the EVAP canister vent control valve) responds to signals from the ECM.
When the ECM sends an ON signal, the coil in the solenoid valve is energized.
A plunger will then move to seal the canister vent. The ability to seal the vent is necessary for the on board diagnosis of other evaporative emission control system components.
When the ECM commands the valve 'closed" but the system does NOT show that it is sealed, the ECM assumes that there is a "Small Leak".
The next step is to make certain that the valve can actually make a seal. Try pinching off the atmospheric inlet hose and repeat the evap test. If it passes, replace the solenoid.