Re: Error downloading large video from iCloud to Mac Studio : "The operation couldn't be completed"
You may want to check that your iPhone and Mac are using the newest compatible OS versions.
Settings (or System Settings) > General > Software Updates will show whether or not updates are available
(Reminder: Backing up the data on a device before updating OS is always a wise move.)
Else: Wondering whether Apple Support's online article (link below) can help:
Check ways to: Download iCloud photos and videos - Apple Support
Also: if needed: The article below has some tips that may be helpful.
Transfer photos and videos from your iPhone or iPad to your Mac or PC - Apple Support