Are you able to save files from any other program to the NAS via the iMac?
The issue of permissions indicated exactly that. It is probably related to hoiw you have your NAS setup. Launch the Control Panel for the NAS in a browser and check over the settings for SMB/AFP/NFS You will need to make sure AFP is enabled.
Next find any folder in the NAS (again from within the browser) and look at the permisions for it. You can have multiple users for the NAS with varying permissions. Make sure the user account use log in to the NAS with has the appropriate permissions.
I am not an expert with Networks but I have been using a Synology NAS for a few years now with zero issues relating to saving a PSD file from Photoshop. I store my images on the NAS, open them directly through PSCC or via Lightroom CC Classic and have no permissions errors.
Thanks for your advice, but the NAS isn't the problem. Everything is filled in correctly. I don't have issues with other files. It's a Photoshop/Mac/Nas issue.
I am having the same issue. I can save and save over any and all files from other apps (except photoshop)... This started immediately after upgrading to OS 10.13.6 from 10.9.x
I’ve been having the same issues with lightroom and photoshop not writing files to my synology NAS due to so called permissions issues and yet other apps are fine.
After much messing around, the solution I found is on the synology go to control panel and file services and ensure SMB is disabled, and AFP is enabled. Restart the synology and your Mac. Now log in to the synology from the Mac with afp://ipaddress
you should now find lightroom and photoshop behaving correctly, saving files without issues.
I am am unsure what the issue is, but there are discussions out there that the macOS and synology SMB protocol don’t behave nicely together.