Hi there! Are you trying to activate that device on a new account? Do you have an active Virgin Mobile account?
Oh, I see. At the moment, only new customers are eligible to bring their own phone to Virgin Mobile, and only iPhones are compatible with this program. You can get more information here at https://www.virginmobileusa.com/plans/bring-your-device.html
You can try activating the device here at https://activate.virginmobileusa.com/#/your-device.