We have previously create many sub-class under one parent class, however, currently we are facing a problem which unable to create the sub-class, how to overcome this?

Hi there, Grace! Thank you for taking the time to post your concern in the Community. We appreciate your query regarding the inability to create the sub-class under the parent class in QuickBooks Online (QBO). We'd be pleased to help you clear things up.

Usage limits may vary on your QuickBooks Online (QBO) subscription. These limits apply to how many billable users, a chart of accounts, classes, and locations you can add to QBO.

In QuickBooks, your subscription determines the specific usage limit for the maximum number of entities, like accounts or users, that you can have at one time. These limits apply to the number of billable users, chart of accounts, classes, and locations you can add to QuickBooks.

If you're using the QBO Simple Start and Essentials versions, the option to create classes isn't available for this subscription. However, subscribing to the QBO Plus version enables you to establish 40 combined classes and locations. Additionally, the QBO Advanced version allows unlimited creation of classes without any limitations.

Here's an article you can refer to for more details about the limits of your QBO account: learning about usage limits in QuickBooks Online. 

I have here resources you can read for additional reference and a guide about the class tracking feature in QBO:

• Create and manage classes in QuickBooks Online.
• Track your transactions by class.

To keep an eye on each segment of your business, please keep this article handy for future reference: Run reports by class. 

I wish you continued success with your business. Please let me know if you have any other questions about setting up classes by commenting below. I'll be around to help you.