Triangle with exclamation mark on mac photos

The ! shows up when Photos cannot find the original images.

Where is your Library stored? If it's on an external it needs to be formatted Mac OS X Extended, Journaled or apfs.

Have you used an apps to "clean" or "tune up" your Mac?

could it be, that your iphone us taking the images in the high efficiency format Heif? A mac eith an older system version cannot handle this format and will show the exclamation point. you need at least macos high sierra.

I think that Leonie is right. If your photos have heic extension you can import them on your Mac from the iPhone using "Image capture" app that you can easily find because it is a System App.

Afterwards -using a lot of free software you can find searching internet (but I presume you can even use Preview)- you can convert heic format photos in jpeg format photos and use your photos in Photo app even if you are still working with a system older than High Sierra.

Anyway -if your Mac is compatible with Catalina- I suggest that -sooner or later- it should be better to upgrade your System to Catalina, al least.