Mail Merge on Max with Word/Outlook

I've been running a Mail Merge successfully on my Windows 10 platform for some years.

I've transferred the files to a MacBook Pro and Office 365.

My first attempt to run Mail MNwrge was unsuccessful as I think I was using the "new" Outlook. I've now reverted to the "old" Outlook".

However when I run the Mail Merge I get this error:

At least one recipient is not valid., Participant has empty SMTP address: [OneOff " email address removed for privacy reasons ", (none):(none), SMTP:(none)]. Mail could not be sent. The message has been moved to your Drafts folder.

When I look at the Draft the email address (which is correct) is highlighted in red.

Is anyone able to interpret the message as I'm really unsure where the problems are. The Mac versions of Word? The merging process? My data?

Thanks for any guidance,

Rod