Can’t receive or text messages on iPad

By itself, an iPad is limited to internet messaging only. It can send and receive iMessages if you sign in to the messaging app in settings with your AppleID - iMessage is Apple's proprietary internet messaging service. Other internet messaging services available through the App Store are Skkype, Google Hangouts and a number of other third party internet messaging services.

SMS/MMS texting requires a cellular telephone as they use the cellular telephone network's voice channel. An iPad is incapable of connecting to any cellular telephone voice network, and thus cannot send nor receive SMS/MMS messages by itself.

If you own an iPhone, you can pair it with the iPad (Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support) so the iPad can share the iPhone's SMS/MMS texting capabilities. But without an iPhone, an iPad cannot send nor receive SMS/MMS texts.