Unfortunately you can’t save save mid-missions, and missions can last forever, have multiple parts, et cetera. You quit – you lose progress. But there’s a way to partially solve the problem.
Save Game Guide
You can manually save but not while you are doing a mission. Some missions auto start when you are near them. Abort mission and save if you need to.
So you can save. But not while you are on a mission.
- Finish or abandon a mission.
- Select save.
Note: You can have several save slots. While on missions the save option is not available and it only will autosave checkpoints.