Hi there Mojopo7,
If you are unable to upload one specific video to iCloud, first check the size of the video by going to the Photos app, selecting this video and then selecting the "i" icon.
If this video's size is smaller than your available iCloud storage, next let's check and clear your Recently Deleted folder since this can also cause this error message to appear. These steps from Delete photos on your iPhone or iPad show how to do this:
Delete photos permanently
1. Open Photos and tap the Albums tab.
2. Tap the Recently Deleted album.
3. In iOS 16, iPadOS 16.1, or later, use Face ID or Touch ID to unlock your Recently Deleted album.
4. Tap Select.
5. Select the photo or video that you want and tap Delete. If you want to delete all the photos and videos in the album, tap Delete All.
6. Tap Delete Photo to confirm.
When you delete a photo from this album, you can't get it back.
Once you've deleted these photos and videos, let us know if this continues to occur and we will assist further.
Cheers!