Your browser is running out of local storage. Chrome has a default limit of 5 or 10 MB. This is too low for Free Cities.
Solutions:
- Use Firefox instead of Chrome. Firefox also has a low limit by default for local storage, but it can easily be increased (Chrome does not allow that). You can type "about:config" in the Firefox address bar, then search for dom.storage.default_quota. Typing the first few letters of this variable should be sufficient for you to find it in the list. Then you can click on the little pencil icon to edit the value. The default is 5120 and you can for example multiply it by 10 to give Free Cities 50 MB of local storage.
- Use the FCHost browser that is distributed with Free Cities. This is a modified version of the Chromium browser that allows Free Cities to use much more local storage. I cannot provide a direct link to it nor to its installation instructions because direct links to Free Cities (pregmod) are against the rules of this forum, but you should be able to find it easily if you search for the right terms.
The autosaves are saved in the local storage of the browser. You cannot access that local storage directly. However, you can use the "Save to disk" button to save the current state of the game to disk. I recommend that you save to disk often, and avoid keeping too many standard saves because they usually make your game slower and consume more of that limited local storage.