Hi 
I am working on Trailhead module: Test your Process. I followed all the steps in creating Process Builder through Trailhead and I am getting below error:
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: []
Can one of you help or let me know, what I am doing wrong.
Thanks