Selecting multiple clips in iMovie to change horizontal video to vertical

Select the first clip in the timeline by clicking on it so it outlines in yellow. Shift-select the last clip. That will group select all the clips from the first clip through the last clip.

If you have made the first clip vertical (by rotating it?) Then you should be able to Edit/Copy it, select the rest of the clips, and then Edit/Paste adjustments to the other clips.

How are you making the horizontal clip vertical. You would need to rotate them, or crop them in the media browser.

— Rich

Possibly an iMovie preference has gone awry. That often is the cause of iMovie acting other than as expected.

Try deleting/resetting 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 you can select multiple clips. Possibly you may need to reboot to set the changes.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

Another thing to try is to place your media in a newly created project and see if everything works properly there.

-- Rich