QB Issue Resolution:

change the name of the vendor to do not use, and make it inactive

then create the customer

the name has to be changed cause QB will not allow for duplicate names

Resolution for Issue 'How to edit an account , erroneously entered as a supplier into a debtor account?' available: Yes (Solved).
Source: Intuit Community forum.

How to edit an account , erroneously entered as a supplier into a debtor account?: 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.

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