None of the suggestions I read on any of these community posts or online helped when I had this problem. I kept getting error messages that the accounts for iMessage and iCloud may be different, even though the AppleID was the same on all my devices and verified that several times. I kept receiving error messages when I tried to turn on iMessage on my iPad, which would not go away unless I did a hard reboot to the iPad. The accounts, i.e., the email addresses, were identical on all the devices. I even tried changing the password for my Apple ID and resetting my iPad to factory settings. Nothing worked. And I followed all the instructions on myriad posts I found online, none of which worked, either. I believe changing the password caused a problem because the iPhone was signed in using the old password and the iPad was signed in using the new one. Or at least I think that was the problem. So I signed out of my Apple account on all devices (go to settings/name/scroll to the bottom to sign out), then signed back in on all the devices serially, merging my data and all the apps to iCloud, and ensured that the AppleID and passwords matched when signing back in. This solved the problem and I can now text/receive messages on all my devices, including my iPad, iPhone, and Apple Watch.
