Quit Photos.
Disconnect the iPhone from the Mac, and completely shut down, and then restart, both devices. When restarting the Mac, however, try booting into Safe Mode (hold shift key, this may take 10-25 minutes, be patient). Safe Mode checks the Mac file system, repairs it if necessary, and also clears out caches. Problems with the file system or caches can cause Photos to not work properly.
After restarting everything normally, if the problem persists, try a different cable. If still a problem, try using Image Capture to import your iPhone images into a folder on your Mac.
https://support.apple.com/guide/image-capture/transfer-images-imgcp1003/mac
Once you have all the iPhone images imported from the phone into that folder, start Photos and you can then import the images from that folder into Photos.
If you still encounter problems importing images from that folder into your Photos Library, make a backup copy of your Photos Library (it is in your Pictures folder normally) and try to repair it: https://support.apple.com/en-us/HT204967