Video in IMovie

Check to confirm that your Photos library is designated as the system photo library in the Photos app General preferences. Otherwise the videos won't display in the iMovie Photos sidebar item.

iMovie won't display videos that are stored in the cloud. So, for example, if you use iCloud Photos and have optimized storage selected for your cloud photos and videos, some of them may not show up until downloaded.

Videos that are in a legacy format, such as .avi may not be recognized either.

If none of the above apply to you, try deleting preferences. 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. Now see if your videos are recognized.

-- Rich

You need to download the video that is in iCloud. Click on its thumbnail in the Photos app and it should download. Then see if it shows in the Photos item in the iMovie sidebar.

At least in Photos 4, that I use, one can Control-click on the video thumbnail and then click on Get Info in the pop up menu. In the Get Info box the video's format will be shown.

You won't need to change the format of the video unless it is in some legacy format. You can change the format by doing a File/Export as Original to your desktop and then convert it to an Mp4/AAC file 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 iMovie.

-- Rich