Porting numbers can be finicky. Especially with a service like Google Voice. If your number is eligible, double-check your account information. If Google Voice doesn't even let you move forward, there are a variety of reasons your number isn't eligible. Google Voice won't let you port a landline number, a number that is owned by your school or your work. This means that you can't directly port your number from one of these services to Google Voice. Unfortunately, the workaround for this is to port your number to another cell phone carrier, wait a while, then port it over to Google Voice (usually three days). It's a hassle, but if it's a number you've had for years it's worth a shot.