QB Issue Resolution:

During the full create a customer process or edit afterwards you can designate a customer to be a sub.

Resolution for Issue 'How do I create a sub customer?' available: Yes (Solved).
Source: Intuit Community forum.

How do I create a sub customer?: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for afterwards
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