Can't send messages with iPad Pro
I found my answer here. The basics of the answer is in these steps.

make sure you're signed into iMessage on your non-iPhone device.

Jump over to your iPhone and confirm you're signed into iMessage and iCloud with the same account here, too.

Still on your iPhone, go to Settings &gt; Messages &gt; Text Message Forwarding.

Flip the switch to enable it on one device at a time.


This worked fine for me.
Apple's relevant support articles on the differences between iMessage and SMS messages as well as the full steps to enable this (with screen shots) are here:

https://support.apple.com/en-us/HT207006
https://support.apple.com/en-us/HT201349