If you are encountering issues with saving the game make sure that The Witcher 3 is on the controlled folder access allowed apps list in your security software.
Well known security software that may be tied with the issue is Windows Defender and Bitdefender.
If you are using Windows Defender:
- Open Windows Defender Security Center.
- Click on Virus & threat protection.
- Click the Virus & threat protection settings option.
- Under "Controlled folder access," click the Allow an app through Controlled folder access link.
- Click the Add an allowed app button.
If you are using Bitdefender:
- Go to the Dashboard of BitDefender
- Protection tab on the left
- Click on Application Access under the Safe Files applet
- Flag the check mark for The Witcher 3