I don't know what is causing iMovie not to open the iPhone 11 videos. What is the frame rate of the video clip? Is the iPhone video recorded with the HDR setting on? Is it in HEVC format? What version of iMovie and operating system are you using?
In any event, the first thing to try is to delete preferences and then see if iMovie will open the video clip. To delete 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. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
If deleting preferences does not cure it, try creating a new project and see if you can import into it and have iMovie open the movie.
Failing that, try converting your troublesome clips to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie.
-- Rich