Syncing Photos from iPhone to MacBook Photos app is not working

You should verify that "iCloud Photos" is enabled on each device, and check that each is operating normally.

On the MacBook Air with macOS 15.3, check in the Photos app, Settings window, iCloud pane, see if "iCloud Photos" is checked. If it is, switch to the "All Photos" view in Library, scroll to the bottom, and check the status line under the counts of photos and videos. It should say "Synced with iCloud Just Now" or some other status.

You don't say what version of iOS you have on your iPhone, so I'll assume it is similarly up to date (18.3 or so).

In the Settings app, scroll to the bottom and click on Apps, scroll to Photos and click on it, make sure the "iCloud Photos" slider is slid right and showing green.

In the Photos app, in the main Photos display, tap on your avatar icon in the upper right corner (might be your initials or an image) to switch to another pane topped by the avatar icon and your name. Just below that should be the counts of photos and videos, and the status line should be below that.

If iCloud Photos is not enabled on one of the devices, enable it.

If the status on one of the device is abnormal, let us know what it says.

In addition to markwm's excellent suggestions, I would say that you should go to iCloud.com to see what is there i the Photos section. I there are pictures missing that you see in Mac Photos, for instance, that would suggest that the Mac is the one at fault. Or likewise with the phone.

Let us know what you find, and maybe we can help further…