iMessage

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. Verify these settings:

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

A little more information would be helpful to better understand the nature of your problem.

Is this a problem with SMS (Text) messaging (green message bubbles), a problem with Apple Messages (blue message bubbles), or both?

What happens when you attempt to send a message? Do you see any errors - and if so, what are are they?

… and that was the next step 🙂

I’m delighted to learn that your problem is now resolved.