You cannot do anything about that. If that recipient is not using an Apple device and iMessage, then your iPad cannot send a text to it.
An ipad by itself is solely restricted to internet based messaging services only. Apple’s free internet messaging service is iMessage and it requires that both sender and recipient be using an Apple device and Apple’s iMessage Service.
An ipad can also use internet services like skype or google hangout. So you and your contact could use one of those.
To send or receive SMS/MMS texts you need a cellular telephone voice network connection, which no ipad is capable of. If you have an iPhone you can pair the iPad to it so the iPad can share the phones SMS texting service (Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch - Apple Support).