Lightroom Classic: Saving metadata when files are on NAS

The Exclamation mark in the upper right hand corner of the image says that LR Can't locate that image. LR Thinks it is Missing.
Is the NAS mounted and showing up in Finder?

I don't believe that's what that exclamation mark means... Yes, the share is mounted and showing up. In fact, I can right click on the image in Lightroom and choose "Show in Finder" and it opens finder AND selects that specific file. I can also do an open in Photoshop from Lightroom and that works. The file is definitely there.

Also I believe that the Adobe warning about a NAS is that you can NOT store the Catalog file on a NAS.
Other than that I have never seen an Adobe warning about storing your image files on a Networked Attached Storage device (NAS).
I have nothing else to add other than I have never run into the use of the Exclamation mark unless LR thinks the image is missing.
Could you explain what it is there for if not for missing images?

It just means there's an error of some kind with that file. In my case, the error is "Error writing metadata," as you can see in the attached screenshot. Of course, if a file is missing, you'll see that same icon and if you mouse over it, it will say "File missing."

Adobe has long stated that many of their creative products, including Photoshop, aren't supported when reading and writing documents on NAS:

https://helpx.adobe.com/premiere-pro/kb/networks-removable-media-dva.html
https://helpx.adobe.com/illustrator/kb/illustrator-support-networks-removable-media.html
https://forums.adobe.com/message/4542869#4542869 (Chris Cox was a very senior Adobe engineer)
https://community.spiceworks.com/topic/1434968-editing-adobe-files-directly-on-a-network-volume-shar...

I've never seen an Adobe help article, blog post, or forum post stating a support policy for using NAS to store LR-cataloged photos. But there have been a few reports of Adobe customer support "recommending" against it:

https://forums.adobe.com/message/9399243 

(We all know how unreliable Adobe technical support can be.)

Overall, the general thrust is that Adobe doesn't want to take responsibility for providing customer support of their creative products on network storage. I understand their rationale, though I don't agree with it. Adobe engineer Chris Cox summarized it as, "Photoshop does not officially support saving to servers, because there are many problems with servers and networks outside our control."

To repeat, I haven't seen any Adobe statement of NAS policy with respect to LR in particular, and I don't agree with their general creative-products policy.