I can't text a particular contact from my iPad Air.
I can't text a particular contact from my iPad Air, but I can from my iPhone X. All other texts go through OK.
iPad Air, iOS 12
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SMS and MMS messaging are cellular carrier features that iPad alone cannot access. Assuming that both your iPad and iPhone are signed-in to iCloud with the same AppleID, 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.
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
More information:
Fault finding if needed:
If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support
I hope you find this to helpful in resolving the issue.