We are so sorry you are experiencing this problem, and our Engineers have been notified.
There is a work around to this while we fix the problem: when creating a new class enter a colon between the parent and the child class.
Resolution for 'adding sub issue: 'What happened to the checkbox for adding sub-class?' available: Yes.
The adding sub issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the adding sub issue yourself by looking for a resolution described in the article. If it is a complex adding sub issue or you are unable to solve the issue, you may contact us by clicking here or by using other adding sub support options.
For more information about resolving issues related to adding sub in Quickbooks/QBO or to get support for the issue 'What happened to the checkbox for adding sub-class?', click the link below: