I have tried numerous times to complete this module - Automate Simple Business Processes with Process Builder with no success.  I've tried it in both my trailhead playground and my developers account.  I keep getting the following error message, even after deleting any process associated with contacts/accounts but still no luck.  Any suggestions would be helpful, thanks!

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: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, Account Number must be 8 characters long.: []