Downloading movies

I meant that no matter where you are saving them to, OSX will 1st use the Boot Drive... it needs plenty of space on your Boot drive to store temporary files.

Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Yes, 121GB of storage is plenty to download some movies. If you are still running MacOS 10.13, you will want to go to iTunes> then choose Movies from the selection menu and there should be a Purchased tab somewhere in that window.

Double tap, or right mouse click open with. It is stored on your iCloud. If you are having connection issues to your iCloud account it won’t work. Your account info for Apple Support Communities should allow you to login iCloud with Apple menu - System Preferences.

How much free space do you have? Most Airs have a limited amount of space for movies. You need a minimum of 5 GB for a single 2 hour movie.

16 GB is not even enough for safe operation, the OS itself needs that much elbow room or more.

I believe OSX uses the Boot drive to download temporary files before saving them to the external drive.