Before you go any further, try deleting preferences. Open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if the black screen is gone. If still no luck, you can consider the things mentioned below.
On the iPhone you can select the recording resolution in Settings/Camara, but I don't know of any way with the iPhone to lower the resolution at the time of export. You can put a test clip into QuickTimePlayer and export it out at a lower resolution. Or, you can put the clip into the Mac Photos app and do a File/Export/Export 1 Video that will give you export resolution options.
However, the error message indicates that the issue is with a frame, which would indicate internal corruption of the clip. The clip is entitled "Opening Credits" so possibly there is some corruption in the credits title template that you chose. You could try changing out the template to a different one.
Also, your iPhone has iMovie for iOS on it. You could put the clip into an iMovie for iOS project where hopefully it will play properly there. Then AirDrop the iOS project itself to your Mac. (The project, not just the video clip.) It will go into your downloads folder. Then, from iMovie for Mac, you can do a File/Import iOS Project, and your iMovie for iOS project will open as an iMovie for Mac project. Sometimes that cures issues with clips. See if your video clip plays O.K. in the converted project.
To cover the bases, iMovie supports 4k 30fps and 1080p 60fps, but not 4k 60 fps. So possibly if you have a 4k60fps clip, or some extra high frame rate on some other resolution clip, that might account for iMovie's difficulty in reading it. Check your frame rate and see what you have.
-- Rich
