Hi Mark,
Most of the times you don't have to worry about this kind of Google search console warning. It means that you have duplicate pages and some of them do not have a canonical link so Google is not indexing them. But the alternate pages (which have the same content) are indexed. So this means that probably you are not losing any SEO value here.
In order to resolve this issue, you need to check the details that Google provides about this error. You need to check which URLs are not being indexed and what are their alternate pages with proper canonical tag.
Route 66 adds the canonical tag all the pages so it may help you remove some of those errors. Route 66 can also redirect those pages to their alternate pages.
You can download the free version and give it a try. Since you are not going to change your URLs you just need to install Route 66 and check that the option named "Canonical links" in enabled under the component options.
Finally note that you will have to give some time to Google in order to see the results in search console. I recommend to wait around one month before checking the errors again.
Regards
