Thanks for the prompt response , Jack!
The iMac is iMac (Retina 4K, 21.5-inch, 2017), OS High Sierra 10.13.6
The iPhone is 6sPlus, Software version 14.5 (updated a few days ago)
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After clicking on "Learn More" I was taken to:
https://support.apple.com/en-us/HT208831
Then followed the link at:
If your Mac doesn't recognize your iPhone, iPad, or iPod touch, learn what to do.
Then followed this link:
https://support.apple.com/en-us/HT204095#Mac
If you see your iPhone, iPad, or iPod under USB Device Tree, get the latest macOS or install the latest updates.
The phone was recognized so I followed the link to:
https://support.apple.com/en-us/HT201541
Choose System Preferences from the Apple menu , then click Software Update to check for updates.
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There is no "Software Update" item in "System Preferences", (perhaps because it's High Sierra.)
(BTW, I don't want to update to the next OS as I have 3 machines, 2 of which are already at their limit.)
BTW2, "Photos" reacted to connecting the iPhone, but showed a photo already deleted from the iPhone as available for upload to the iMac.
TIA