Before doing anything, I would suggest that you contact Apple Support and have them walk you through this. They have the capability of remotely accessing your computer that we cannot do here. To contact support click on the Contact Support item in the upper right hand corner of this forum's screen and navigate to a place where you can enter your
phone number.
If you prefer to work on it yourself, you can try converting the incompatible clips to H.264, Mp4/AAC with the free download, Handbrake.
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 your iMac or into iMovie on your iMac.
I would try it with one test clip before converting them all. If you wish to preserve the original format of your clip you can make a duplicate of the clip and convert the duplicate.
iMovie 8 will not work with Catalina because iMovie 8 is a 32 bit application.
If you have the iMovie 10 app on your MacBook that is running El Capitan you may want to open iMovie 10 and do a File/Update Projects and Events. That will create a special library in iMovie 10 where your iMovie 8 projects will be updated and then can be played and edited in iMovie 10. You can still keep iMovie 8 and its projects on your Mac as well, if you wish.
Also, if you can update to Mojave 10.14.6 on your MacBook you can then update your iMovie 10 and do the conversion there.
However, since you have said that you are not super experienced with these kinds of procedures, I reiterate my earlier suggestion that you first contact Apple support.
-- Rich
