It is corrected by having that user register and set up messages on his or her device. If you are trying to message someone that has not set messages up on their device, you can't reach them via the Messages app. They have to set up and register to use Messages first.
Messages only works with other iOS users. You cannot message with an Android user or any other cell phone either.
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID