These items cannot be added to your Photo library because the file format is not supported by Photos.
Are the filenames of the items shown, that you cannot import? what is the filename extension?
Have you tested, if the Image Capture application can download the items to a folder on your Mac?
This would allow you to convert them to a different format.
Where is your Photos Library stored? Sometimes the file system of an external drive is the problem.
What is the model of your Mac? Even with High Sierra installed, you may not be able to open all videos from an iPhone X on your Mac, if the Mac is too old. See this document: Using HEIF or HEVC media on Apple devices - Apple Support