Why Can’t I import photos to iPhoto on a desktop?

Your photo is showing that you are trying to import to Photos, not iPhoto.

Is iCloud Photos enabled on your iPhone? In that case the "preparing" may never stop.

You may want to check the Settings > Your Name > iCloud > Photos on your iPhone. If iCloud Photos is enabled, you should use iCloud to download the photos from the iPhone.

Which System version is running on your Mac?

And is the system version given below your question "iPhone 6, iOS 10" still valid?

Next time take a screenshot instead of photographing the screen. It makes for a better image and easier to make and post.

How to take a screenshot on your Mac 

(⌘⬆︎3) Command-Shift-3         Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4         Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5         Invoke new screen grabber

Try another port on your Mac.

What systems are you running on your Mac and iPhone?

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and and try to import while there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Hi - how are you connecting? Presumably USB.

Can you try another cable - particularly the Apple original.

How long have you left it preparing?