Hello Irishdavid14,
The article that was linked to your posting (Move content to a new Mac) explains that you should take these steps to prepare for the Migration Assistant process:
- "Update the software on both computers.
- Make sure that both computers have Wi-Fi turned on and are near each other. If either of them is using OS X El Capitan or earlier, make sure that they're on the same Wi-Fi network.
- If you installed antivirus or firewall software, turn that software off until you're done."
Since the macOS version on your original Mac is a more recent update that the one on the new Mac, you will need to set up the new Mac as a new device and update it fully. Then you can restart the whole Migration Assistant process and transfer your content.
Let us know if we can help with anything else at all or if this doesn't settle your issue so we can look into this further with you. Thanks and have a fabulous day!