I’ve just downloaded Manager.io and set up a basic chart of accounts. However, I can not find out how to set up subaccounts for income and expenses. I also can’t find how to close the company when finished. I’ve been using Quickbooks since 1998 but have just found that my data has not been saving outside of the program since January and my files refuse to backup. Thus I’m looking for a good alternative program.
check out:
https://www.manager.io/guides/design-a-chart-of-accounts/8965
but the basics are you want to create a group (which is like a folder or collection) and then you create your New Account under that.
Whenever you click on a Create or Update button, Manager saves your data. So you can safely close the program at any time. 
In addition to the guides and instructions provided by @d3mad, I found it helpful to study the Chart of Accounts of the sample business Northwind.
A more comprehensive Guide on actually creating the chart of accounts you have designed is this one: Build a chart of accounts | Manager.
As for backing up data, despite the fact that Manager saves every entry or edit as @dcVest described, you should always maintain separate backups of data for every company on a different drive against the inevitable day when the drive fails. See this Guide: Backup, restore, import, and transfer businesses | Manager.
both of those links are 404’ing 
@d3mad try again. Links fixed.