What is the file format of the photos on your iPhone?
is your iPhone taking the photos and videos in the space saving HEIC format? The newer iPhone models could be using an image format that your Mac cannot read, if it still running macOS 10.12. In that case you should change the settings on your iPhone, so the images are converted to JPEGS, when you download them.
See: Using HEIF or HEVC media on Apple devices - Apple Support
When you import HEIF or HEVC media from an attached iOS device to Photos, Image Capture, or a PC, the media might be converted to JPEG or H.264.
You can change this import behavior in iOS 11 or later. Go to Settings > Photos. In the TRANSFER TO MAC OR PC section, tap Keep Originals to prevent the media from being converted to JPEG or H.264 when importing.
You can also change the settings the iPhone, that the photos and videos will be taken in a more compatible format. or upgrade the system on your Mac to macOS 10.13 or macOS 10.14, if your Mac can be upgraded to this system.
