I am running LRc 9.4 and using a Mac, High Sierra (10.13.6). I previously had an iPhone SE and imported without any problems. When I import it states that, 'following files not imported because they cannot be read or the destination folder is not wrtitable'. I am importing from an iPhone X (16.0.3). I have changed in in the iPhone settings/camera capture to high efficiency. What am I doing wrong?
Using Mac Finder, check the file system permissions on the destination drive and destination folder as listed in the LrC import screen. Sometimes, just opening that section of the import dialog fixes the problem.
Also check the security permissions for LrC in the Mac security program.
I have done what you suggested but to no avail. Is it because my old version of Mac which cannot take updates from LRc and/or is incompatible with iPhone X?
How do you import the images? Do you perhaps use the built-in camera of Lightroom mobile and sync? The iPhone X is supported as camera for a long time, so your version 9.4 should support it. Syncing between Creative Cloud and Lightroom Classic requires version 10.4 minimum, however.
Then I see no reason why Lightroom would not import the photos, unless you use Apple iCloud Photos and the iPhone has uploaded the photos to iCloud and deleted the full local copy.
High Efficiency is HEIC or Apple ProRAW. Lightroom Classic 9.4 may not support that. Another thing to check is Preferences - Lightroom Sync in Lightroom Classic. Check what destination folder is specified here. The folder must be inside your Home folder, otherwise it may be off limits to Lightroom Classic.