Share messages across all apple devices

Are you attempting to send/receive SMS/MMS (green bubbles) messages from devices in addition to your iPhone? To do so, assuming that all your devices are signed-in to iCloud with the same AppleID - to send SMS/MMS messages, you must explicitly enable Text Message Forwarding for your other devices on your iPhone...

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable each device from the list.

Also, to keep all devices synchronised, ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON