If you’re looking for a way to save your game, there’s one underneath the interface, towards the bottom of the screen. If you’re looking for a way for players to save their progress, it’s not supported now, but it’s on the development list!
https://trello.com/b/5i4VvOW8/rpg-playground-roadmap
Here’s the link to deWitter’s to-do list. You can upvote ideas you want him to prioritize or hop on the discord to suggest something for him to add to the list.
Best of luck on your game!