Exporting photos

The "Delete items" checkmark is not shown for all camera models (Import photos from a camera or phone using Photos on Mac – Apple Support (UK)).

Do you have iCloud Photos enabled on the iPhone? Then the option to delete after import will not be available, because the photos are primarily stored in iCloud and not on the iPhone.

But I would not delete the photos automatically from the device anyway. It is very risky. It is much safer to check the imported items after the import, if the photos have been transferred correctly without any transmission error, and only then delete them from the camera or iPhone. When you delete the photos from the iPhone before you have checked them, if all have been transferred correctly, you will be unable to repeat the import, if something went wrong. Occasionally the download may go wrong and you would lose precious photos or be stuck with damaged copies with artifacts, if you let the Mac delete the originals, before you have inspected the transferred photos. It is better to wait until you have checked the imported photos an made the first backup, before you remove them from the iPhone.

Hier is current example of what can happen, when using the "Delete items" option, either in Photos or in Image Capture: Image Capture lost my pictures - Apple Community - thousands of pictures lost.