Some images could not be exported because we couldn't access the full size original

It's not going to help much as Adobe made the decision to hamper syncing from Classic many years ago but always good to add your voice here: https://community.adobe.com/t5/lightroom-classic-ideas/p-should-be-able-to-sync-full-raw-files-to-the-cloud-not-just-smart-previews/idi-p/12248616

Hi, in case it helps, I found a way around this nonesense made by Adobe. Because it hasn't been solved. It seems it has to be with the way lightroom exports photos. So when it tries to look for the folder with your originals and can't find it it cannot export any photo even if it's backed up on CC. So, what you should do is uninstall lightroom and/or Lightroom Clasic, then (on Windows) go to %appdata% on a run promt, search for any Adobe Folder and delete every Lightroom file and folder you see on the Roaming folder. The go back to the /Local folder and do the same for any file or folder you can find about Lightroom. Only then you can go and reinstall lightroom with CC launcher. That's it, it solves the issue, now lightroom will be able to download your originals from the cloud because what you did is help lightroom repath the Export process and the folders it was linked to. Hope it helps even tho I'm not really good at explaining. Adobe fix this. 

I was not aware of the difference between the two unfortunately. I downloaded ~1400 of my wedding photos and began editing them on-the-go on my iPad and on my phone (whittling it down to around ~600 photos). (Long story is that the wedding photographer was unable to assist with this due to a personal emergency.) The original PC that contained all of these photos is very old and had Lightroom 3 on it and I uninstalled Lightroom on it. After reading this forum, I have reinstalled it Lightroom 3.6 on my laptop. My computer is too old to install Lightroom Class (doesn't meet requirements). How do I sync all the work I have done to the original sized photos? Or do I need to re-do all the work I have done? Many, many thanks to everyone for helping me on this!

You are getting it because, as you said, "The photos are all synced and are online and available from my PC via Lightroom Classic". So they are synced as smart previews, meaning that LRCC can't access the full size original and generates that rather stupid and unnecessary message.

You would not get it if you synced originals via LRCC or via a mobile app.