iMessages is not syncing between the Mac, ipad and iPhone.

The Messages app on an iPhone handles iMessages, SMS messages, and MMS messages. Macs and iPads don't have cellular text message service. So if you're trying to send and receive SMS and MMS messages on your Mac and iPad, they need a little help from your iPhone:

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

There's no syncing of any messages? Whether iMessages or SMS/MMS messages?

Do both the iPhone and the iPad have a connection to the Internet? iMessages travel over the Internet, so if either device's Internet connection is down, attempts to send messages may time out.

The iOS version is not the issue here.

All devices must be on the same Apple ID.

Toggle iMessage off and then On on your iPhone.

No, you're not talking to a bot.

You're not talking to paid support staff either – just another user, like yourself.