Unable to send messages to non Apple devices from IPad

To send or receive SMS or MMS messages an iPad must be paired with an iPhone - Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support

SMS and MMS require a cellular telephone voice channel connection, which no iPad is capable of. So by itself, an iPad is limited to internet messaging services only, like iMessage, Skype, Google Hangouts and others.

In addition to what Michael posted, you must have an iPhone. If you are Android user, you won't be able to use continuity services to be able to use texting to non-iOS devices. In that case, you would be limited to text messages with iOS devices only.