trying to save my iMovie presentation for school, but it keeps saying that the download failed

Hi, Giraffe_Girl,

Are you getting any error message? If so, what is the full text?

Check to make sure that you have sufficient application space on your main drive, and sufficient free space on your destination drive.

You might also scroll through your project looking for signs of corruption that could cause a export to fail. Look for white flashes, black frames, and artifacts that would indicate corruption. Cut out any that you find. If you cut out things it might be good to work with a duplicate project so as to preserve your original.

If still no success, here are some diagnostic things that you can 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.

Opening iMovie in a new library

Try opening iMovie in a new library.  To do

that, open iMovie while holding down the Option key and then selecting to

create a new library in the box that appears.

Opening iMovie in a new user account

Create a new user account from System Preferences/Users and Groups and try to open and

use iMovie in the new account. You can use your Mac's Guest account for this purpose.

New Project

Another thing you can try is to copy your media into a newly created project and see if

that cures the problem. To do that, do an Edit/Select All, Edit/Copy, of the

media in your project and then Edit/Paste it into the newly created project.

Safe Mode

You could also try booting up in Safe Mode (see Finder menu for instructions) and,

after everything loads, immediately shut down and reboot in normal mode. Then

see if iMovie works properly.

Updating iMovie

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications

folder into the trash, but do not empty the trash. Sign in to the app

store and open your purchases folder by clicking on your name at the bottom of

the app store side bar. You will see your purchases displayed, including iMovie 10.

Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the

trash and return it to the Applications folder.

Resetting the System Management Controller

Try resetting the System Management Controller. It is a matter of rebooting while holding down certain keys.

https://support.apple.com/en-us/HT201295

 Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM, although this issue would be fairly rare:

https://support.apple.com/en-us/HT204063

-- Rich