Created new document library and enabled 'Allow management of content types'. Created a new document content type, assigned columns to the CT. I've not done anything fancy or unusual. I've been using this video as guidance.
I used these settings for the content type:
I think the problem comes when I am trying to set a Word template to my new content type - I used this other video for guidance.
I've tried swapping the associated template doc from one with Quick Parts to one without, but it made no difference - same error.
Here are the error messages I see when I click on 'New Doc' in the ribbon and select the new template:
Update 1: I found a similar post with a good answer. I have done all of the steps shown is this post, then I get the error message.
A 2014 post which has the same error.