Quick summary

The drive with your photos is full, so Imagen couldn't save your edits.

How to fix this

1. Free up space on the drive where your original photos are. If they're on an external drive, free up space on that drive, not on your computer. For example, delete files you don't need, or move other files to another drive.
2. Mac: Files you delete from an external drive stay on it until you empty the Trash. Empty the Trash while the drive is connected.
3. Don't move the photos you uploaded to Imagen. Imagen needs them in the same place to save your edits.
4. Click Try again in the message.