If you have an iPhone - and both iPad and iPhone are signed-in to iCloud with the same AppleID, to send SMS/MMS messages from your iPad you must explicitly enable Text Message Forwarding for your iPad on your iPhone...
On your iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
Settings > Messages > MMS Messaging - set to ON
Also ensure that Messages in iCloud is enabled on your iPhone and all iPads:
Settings > [Your Name / AppleID] > Messages - set to ON