This error means your message includes a recipient name variable but is going to more than one person on a shared thread. To fix it, do one of the following:
Turn on Send separately so each recipient gets their own thread with their name filled in. This works when every recipient is a saved contact.
Send to one recipient at a time.
Remove the recipient name variable from the message.
Note: If any recipient is a typed email address that isn't a saved contact, Send separately won't resolve the variable for that person. Send to them without the variable, or add them as a contact first.