Getting the following error and cannot complete Trailhead challenge.

Challenge Not yet complete... here's what's wrong:
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Delete failed. First exception on row 0 with id 00141000013JMSvAAO; first error: DELETE_FAILED, Your attempt to delete TestMyCodeCo could not be completed because it is associated with the following cases.: 00001055 : []