Mojave Error9956 NO Importing

Try the following in order as needed:

1 - as a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? This tells us if the problem is limited to your current library or is more wide spread. If successful then test again with your original library.

2 - boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

2 - launch Photos with the Option+Command keys held down to repair your library.

Got one more trick: log into a new, basic, admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. This tells us if the problem is limited to your user account or is system wide.

Hi - Yes, they should

We are not apple here, we are users like you - if you want to give feedback to apple, the only channel they might listen to is:

Feedback - Photos - Apple

They are more likely to do something if a lot of people request the same.

No it won't. The image size, i.e. pixel dimensions, will be the same. The only difference will be the size of the image file.

Try it with a few test photos and see if that will allow you to import. If it does you can then run a test with photos of the same subject with HEIC and JPEG and compare them.

Mojave should be able to handle HEIC photos. However, if distributing them to others or on the web you'll have to convert to JPEG. The web can't handle HEIC and only Macs with High Sierra or newer can.

Make sure in the iPhone's Settings/Camera/Formats pane the "Most Compatible" setting is selected.

Don't expect a fix any time soon. Mojave is 3 systems (4 counting the new beta Ventura) old and I doubt if any fixes, other than security types, will be released. But we can always hope.

True - but the fix should be in IOS, not in Mojave.

Apple should not be breaking older OS systems for no good reason.