Q: Why can't is send text messages to non-Apple devices on my iPad Pro?

There is no solution in the works because iMessage functions the way it is designed to function. iMessage is an Apple only app that works over WiFi with other iOS devices and Mac computers. It was never intended to work with Android phones. You can use text message forwarding on an iPad to text with non iOS users, but you still need an iPhone in order to do that.

Hope this helps:

How to set up and use SMS/MMS relay to send and receive texts on your iPad

http://www.imore.com/how-to-set-up-use-sms-mms-relay-send-receive-texts-ipad

Do you have a cellular text plan directly on your iPad, or at least an iPhone with a cellular text plan that can be linked to the iPad for sending the messages?

If you have an iPhone use Continuity to connect it to your iPhone - Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support

SImple reality is that SMS/MMS texts require a device capable of a cellular telephone voice connection, while iMessage does not.  iMessage is an Internet messaging service offered by Apple.  SMS/MMS texts is a non-internet, cellular voice messaging service offered by cellular voice service providers.  Since an iPad is only capable of an Internet connection over a cellular networ or wifi, it cannot make any use, natively, of any cellular voice services.

With iOS continuity, your iPhone can handle the actual sending and receiving of SMS/MMS texts, and share those with your iPad over your local wifi connection.

As far as iMessage and non-Apple devices - iMessage is Apple's free service for their product customers, not other companies customers.